ihandle: Add FDH_ISUNLINKED

Add the FDH_ISUNLINKED functionality to ihandle. This lets the caller
know if the file for the underlying file descriptor has been deleted
out from under us. This is useful for sanity checks in some callers.

Reviewed-on: http://gerrit.openafs.org/8838
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 9cf9a0e978ece2b0afb8ba5947455f307a424cab)

Change-Id: If5cde825a2e7413c47409c69f15507d6df1934c3
Reviewed-on: http://gerrit.openafs.org/9507
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: BuildBot <buildbot@rampaginggeek.com>