9p: add support for sticky bit

GDM gets unhappy if /var/gdm doesn't have the sticky bit set.  This patch adds
support for the sticky bit in much the same way setuid/setgid is supported.

With this patch, I can launch X from a v9fs rootfs (although I quickly run out
of fds in the server once gnome starts up).

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Acked-by: Eric Van Hensbergen <ericvh@gmail.com>