Currently, --ip=dhcp is equivalent to --ip=none
and --ip6=dhcp does nothing either, except for parsing correctly

If the container has no IP address (because it will be assigned via DHCP later),
setting up a default route fails with a warning message.

While this is harmless, the default route should be omitted instead.

When dhclient is used to assign and IP to the container,
it should be able to overwrite resolv.conf

Therefore, we do the /etc mirroring similarly to the situation with
manually configured nameservers.

When both DHCP and manually set nameservers are in use,
warn that the manual ones will be overwritten

Refactored sbox_run to pass the varargs argument list as an array to an
auxiliary function.
The auxiliary function allows running programs with dynamically built
argument lists.

The new capability filter SBOX_CAPS_NET_SERVICE allows forked processes
to bind to low ports (privileged network services).

Because dhcp clients require both low ports and network administration
privileges, this patch also allows (bitwise) combination of capability filters
(except SBOX_CAPS_NONE, which completely drops any capabilities)
to grant both SBOX_CAPS_NETWORK and SBOX_CAPS_NET_SERVICE to a dhcp client.
This way, fnet and fnetfilter calls still do not get CAP_NET_BIND_SERVICE.

* In order to ensure that network interfaces are already configured when
  the sandboxed launches, we run dhclient in forking mode (no -d switch),
  which makes the dhclient command exit when it successfully acquired a lease.
  The dhclient daemon process keeps running in the background.
* We read the pid file for dhclient to find out the pid of the daemon process.
  Because dhclient only writes the pid file in the child process potentially
  after the forking parent process exits, there is some handling for possible
  race conditions.
* All lease files and pid files are under /run/firejail/dhclient/
* The v4 and v6 dhclient has a separate lease as recommended.
* The v4 client is set to generate a DUID, which is also used by the v6 client
  so that the server can associate the two leases if needed.
* /etc/resolv.conf is created in the sandbox just like with the --dns option,
  by mirroring /etc. When DHCP is used, /etc/resolv.conf is normally empty so
  that dhclient can overwrite it the nameservers from the DHCP server.

Current limitations:
* The dhclient processes in the background are not terminated properly
  (by SIGTERM or dhclient -x), nor is the DHCP lease released (by dclient -r).
  The reason for this is that firejail drops all capabilities and privileges
  before the application in the sandbox is launched, which makes it impossible
  to launch dhclient to release the lease or kill the dhclient processes still
  running with the effective user id of root. Instead the dhclient daemons
  die with the sandbox. According to the dhclient man page, releasing the lease
  is not required by the DHCP specification, so this is not a problem, however
  some ISPs may require releasing leases.
  A possible workaround would be to fork another process upon sandbox
  initialization that invokes dhclient -r when the sandbox is ready to exit.
  This would require communication with the main firejail process through
  a pipe, while keeping and required privileges. As this would add some
  complexity but the benefits have limited applicability (compatibility with
  esoteric DHCP server configurations), I chose not to implement this.
* When only an IPv6 address is requested, the interface may possible not have
  a link-local address when we run dhclient. This causes dhclient -6 fail,
  since DHCPv6 uses link-local addressing instead of layer 2 addressing,
  see e.g., https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=783387
  In a future commit, waiting for a link-local address will be added.

dhclient -6 fails if the interface to be configures has no link-local address.
This is especially problematic when only DHCPv6 is used
(e.g., --ip=none --ip6=dhcp), because the wait for a DHCPv4 lease is usually
ample time for the LL address to become available on the IPv6 link.
The LL address must not be tenative.

Therefore, this patch implements waiting for a non-tentative link-local
address in fnet for DHCPv6 configured interfaces.

The command fnet waitll <if> waits for an LL address on the interface <if>.

Currently, the maximum waiting time is 30 seconds,
and the kernel is polled through rtnetlink every 500 milliseconds.
These values seem sufficient for virtual bridged networks,
e.g., libvirt NAT networks.