MFC: r284589
Add the ability to detect ZFS and GELI encrypted file systems to fstyp(8)

MFC: r284644
Fix GCC Warnings

MFC: r284728
Only build ZFS support in absense of WITHOUT_ZFS

MFC: r285426
Remove excess copyrights

MFC: r286569
Use GELI sentinel constant

MFC: r287937
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.

MFC: r292757
Fix order of includes in usr.sbin/fstyp/zfs.c

MFC: r292829
Eliminate unneeded includes.