Add (basic) tcsh completion support

This will only suggest zfsnap commands and ZFS pools/filesystems. It will not
suggest zfsnap flags.

tcsh completion support is quite limited, and the lack of functions
doesn't help either. I'm sure it is somehow possible to support flags
(for both commands and top-level zfsnap), however, I will leave that to
someone else whose tcsh-foo is superior to mine.

This, in my opinion, supports the key completion feature desired: ZFS fs
suggestion.

Works toward issue #51.
License should be first comment
Update NEWS file