makefs: Honor -T timestamps when creating images from mtree manifests

makefs backends rely on the fsnode structure to derive most
of the information about the underlying filesystem objects.
Depending on how the image is built, the fsnode structures
are initialized in the walk_dir or read_mtree functions.
However, read_mtree fails to take timestamps passed by -T into account,
leading to nonreproducible images in backends that do not check for -T.
Fix this and make -T backend-agnostic by adding an appropriate check
in read_mtree_keywords while making sure that mtree entries can
still override -T timestamps.

PR: 285630
Sponsored by: Klara, Inc.
Sponsored by: The FreeBSD Foundation
Reviewed by: markj, emaste
Differential Revision: https://reviews.freebsd.org/D49531