RFE: allow running without IPv6 support

[WOnder93]

Detect if IPv6 is disabled and only test IPv6 stuff if it is enabled.

This allows to run the testsuite also on machines where IPv6 is disabled
via kernel command line (ipv6.disable=1).

[stephensmalley]

If I boot with ipv6.disable=1 and run with this patch applied, I get the following:
``inet_socket/test ............ 34/38 ip6tables v1.8.2 (legacy): can't initialize ip6tables table security': Address family not supported by protocol Perhaps ip6tables or your kernel needs to be upgraded. ip6tables v1.8.2 (legacy): can't initialize ip6tables table security': Address family not supported by protocol
Perhaps ip6tables or your kernel needs to be upgraded.
Could not open socket to kernel: Address family not supported by protocol
Could not open socket to kernel: Address family not supported by protocol
Could not open socket to kernel: Address family not supported by protocol
ip6tables v1.8.2 (legacy): can't initialize ip6tables table `security': Address family not supported by protocol
Perhaps ip6tables or your kernel needs to be upgraded.
Could not open socket to kernel: Address family not supported by protocol
Could not open socket to kernel: Address family not supported by protocol
Could not open socket to kernel: Address family not supported by protocol
Could not open socket to kernel: Address family not supported by protocol
inet_socket/test ............ 38/38 ip6tables v1.8.2 (legacy): can't initialize ip6tables table `security': Address family not supported by protocol
Perhaps ip6tables or your kernel needs to be upgraded.
ip6tables v1.8.2 (legacy): can't initialize ip6tables table `security': Address family not supported by protocol
Perhaps ip6tables or your kernel needs to be upgraded.
sctp/test ................... Server socket: Address family not supported by protocol
Client connect: Connection refused
sctp/test ................... 1/86

Failed test at sctp/test line 102.

Client connect: Connection refused

Failed test at sctp/test line 107.

Client sctp_recvmsg: Transport endpoint is not connected

Failed test at sctp/test line 112.

Client socket: Address family not supported by protocol

Failed test at sctp/test line 117.

Failed test at sctp/test line 124.

socket - server: Address family not supported by protocol

Failed test at sctp/test line 143.

socket - server: Address family not supported by protocol

Failed test at sctp/test line 148.

socket: Address family not supported by protocol

Failed test at sctp/test line 157.

socket: Address family not supported by protocol

Failed test at sctp/test line 161.

sctp/test ................... 12/86 Server socket: Address family not supported by protocol
Client connect: Operation now in progress
sctp/test ................... 13/86

Failed test at sctp/test line 217.

Client connect: Operation now in progress
sctp/test ................... 14/86

Failed test at sctp/test line 222.

sctp/test ................... 15/86 Server socket: Address family not supported by protocol
Client socket: Address family not supported by protocol
sctp/test ................... 16/86

Failed test at sctp/test line 238.

Client socket: Address family not supported by protocol

Failed test at sctp/test line 243.

Failed test at sctp/test line 248.

Failed test at sctp/test line 253.

Failed test at sctp/test line 258.

Server socket: Address family not supported by protocol
Client connect: Operation now in progress
sctp/test ................... 21/86

Failed test at sctp/test line 277.

Failed test at sctp/test line 282.

sctp/test ................... 65/86 netlabelctl: error, no message was received
netlabelctl: error, invalid argument or parameter
Server socket: Address family not supported by protocol
Client socket: Address family not supported by protocol
sctp/test ................... 66/86

Failed test at sctp/test line 661.

Client socket: Address family not supported by protocol

Failed test at sctp/test line 666.

Client socket: Address family not supported by protocol

Failed test at sctp/test line 671.

Client socket: Address family not supported by protocol

Failed test at sctp/test line 676.

Failed test at sctp/test line 681.

Server socket: Address family not supported by protocol
Client socket: Address family not supported by protocol
sctp/test ................... 71/86

Failed test at sctp/test line 696.

Client socket: Address family not supported by protocol

Failed test at sctp/test line 701.

Client socket: Address family not supported by protocol

Failed test at sctp/test line 706.

Failed test at sctp/test line 711.

Failed test at sctp/test line 716.

Server socket: Address family not supported by protocol
Client socket: Address family not supported by protocol
sctp/test ................... 76/86

Failed test at sctp/test line 733.

Client socket: Address family not supported by protocol

Failed test at sctp/test line 738.

Failed test at sctp/test line 743.

netlabelctl: error, no message was received
ip6tables v1.8.2 (legacy): can't initialize ip6tables table `security': Address family not supported by protocol
Perhaps ip6tables or your kernel needs to be upgraded.
Could not open socket to kernel: Address family not supported by protocol
Could not open socket to kernel: Address family not supported by protocol
Could not open socket to kernel: Address family not supported by protocol
Could not open socket to kernel: Address family not supported by protocol
Server socket: Address family not supported by protocol
Client connect: Operation now in progress
sctp/test ................... 79/86

Failed test at sctp/test line 764.

sctp/test ................... 80/86 Client socket: Address family not supported by protocol

Failed test at sctp/test line 774.

Failed test at sctp/test line 779.

Server socket: Address family not supported by protocol
Client connect: Operation now in progress
sctp/test ................... 83/86

Failed test at sctp/test line 790.

sctp/test ................... 84/86 Client socket: Address family not supported by protocol

Failed test at sctp/test line 800.

Failed test at sctp/test line 805.

ip6tables v1.8.2 (legacy): can't initialize ip6tables table `security': Address family not supported by protocol
Perhaps ip6tables or your kernel needs to be upgraded.

Looks like you failed 37 tests of 86.

sctp/test ................... Dubious, test returned 37 (wstat 9472, 0x2500)
Failed 37/86 subtests

[WOnder93]

Hm, I don't remember how I tested this patch back then... I'll need to have a fresh look at it.