touch: Allow setting the timestamp to -1.

Note that VFS internally interprets a timestamp of -1 as “do not set”,
so this has no effect, but at least touch won't incorrectly reject the
given date / time (1969-12-31 23:59:59 UTC) as invalid.

While here, fix some style issues.

MFC after: 1 week
Reviewed by: allanjude
Differential Revision: https://reviews.freebsd.org/D44504