Add the ability to detect ZFS and GELI encrypted file systems to fstyp(8)

Fix GCC Warnings

Only build ZFS support in absense of WITHOUT_ZFS

Remove excess copyrights

Use GELI sentinel constant

Eliminate unneeded copying of vdev data, goto, etc. and add a note
that checksum of vdev label should be checked (which is not done
currently).

No functional change.

While I'm there, raise WARNS to 2.

Fix order of includes in usr.sbin/fstyp/zfs.c

Eliminate unneeded includes.