- Fix file descriptor leak in path_to_fshandle. It now caches

- Fix file descriptor leak in path_to_fshandle. It now caches
one descriptor per filesystem.
- In xfsrestore, always allocate a fs handle during content_init.
This allows us to not have to call path_to_fshandle before attempting
every open_by_handle.
- Change open_by_fshandle to open_by_handle is cases where we are
passing a file handle (instead of a fshandle).
path_to_fshandle was opening a file and never closing it. We now close it
unless the descriptor is to be used in the descriptor cache. One descriptor
is cached for each unique filesystem that path_to_fshandle is called on.