makefs: add -O (offset) option

NetBSD revs:
ffs.c 1.60
makefs.8 1.44
makefs.c 1.48
makefs.h 1.33
ffs/buf.c 1.20
ffs/mkfs.c 1.27

Obtained from: NetBSD
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D10780