Added direct_io option

[sunwire]

This option disables the use of page cache in kernel.
This is useful for example if the file size is not known before reading it.
For example if you mount /proc dir from a remote host without the direct_io
option, the read always will return zero bytes instead of actual data.

[root@local ~]# sshfs root@remotehost:/proc /remote/
[root@local ~]# cat /remote/uptime
[root@local ~]# fusermount -u /remote
[root@local ~]# sshfs -o direct_io root@remotehost:/proc /remote/
[root@local ~]# cat /remote/uptime
2280959.53 2221494.84
[root@local ~]# fusermount -u /remote

[Nikratio]

Thanks!