libfuse has an auto_unmount option which, if enabled, ensures that
the file system is unmounted at FUSE server termination by running a
separate monitor process that performs the unmount when that occurs.
(This feature would probably better be called "robust auto-unmount",
as FUSE servers usually do try to unmount their file systems upon
termination, it's just this mechanism is not crash resilient.)
This change implements that option and behavior for glusterfs.

This option will be only supported for clients with version >3.11.
Signed-off-by: Humble Chirammal <hchiramm@redhat.com>