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

Functionality is hidden behind the -u flag to avoid confusing automounters

PR: 200823
Reviewed by: asomers, eadler, wblock (man page)
Approved by: trasz
MFC after: 10 days
Relnotes: yes
Sponsored by: ScaleEngine Inc.
Differential Revision: https://reviews.freebsd.org/D2045