makefs: Provide a -T option to set timestamps to a consistent value

This is taken from the NetBSD versions listed below and adapted to the
makefs version in FreeBSD, along with a bug fix from cem@ that will be
sent to NetBSD.

Reviewed by: pfg
Approved by: re (gjb)
Obtained from: NetBSD
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D6835