newfs_msdos: add -T (timestamp) option for reproducible builds

This includes some whitespace and minor bug fixes relative to NetBSD,
which will be submitted upstream at the conclusion of the makefs
msdos update.

NetBSD revs:
mkfs_msdos.c 1.11
mkfs_msdos.h 1.4
newfs_msdos.8 1.22
newfs_msdos.c 1.44

Submitted by: Siva Mahadevan <smahadevan@freebsdfoundation.org>
Reviewed by: emaste
Obtained from: NetBSD
Sponsored by: The FreeBSD Foundation