ceph-fuse: test dentry invalidation options and fail out if we fail

ceph-fuse: test dentry invalidation options and fail out if we fail

We identify the Linux kernel version and based on that either expect to
be able to invalidate dentries effectively, or expect to be able to remount
the ceph-fuse mountpoint. Test it using the Client functions and callbacks by
spinning off a thread to invoke the test that is separate from the main
FUSE loop.

Most unfortunately, there doesn't seem to be a good interface to tell
FUSE to shut down if we need to do that. See
http://fuse.996288.n3.nabble.com/libfuse-exiting-fuse-session-loop-td10686.html
I tried changing our signal invocation or attempting a simple action on
the mount point but those were ineffectual at terminating the remaining
processes; fusermount actually gets rid of them all.

Signed-off-by: Greg Farnum <gfarnum@redhat.com>