fsrvp: prune shadow copies if associated path doesn't exist

fsrvp: prune shadow copies if associated path doesn't exist

This patch implements some simple FSRVP server housekeeping. On startup
the server scans the cached entries, any entries where the underlying
system paths associated with shadow copies no longer exist are removed
from the cache and from the registry.

This behaviour is disabled by default, but can be enabled via the new
"fss: prune stale" smb.conf parameter.

Signed-off-by: Noel Power <noel.power@suse.com>
Signed-off-by: David Disseldorp <ddiss@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>