When processing mtree(5) MANIFEST files, makefs(8) throws will throw an
error if it encounters an entry whose "time" attribute contains a
non-zero subsecond component (e.g. time=1551620152.987220000).
Update the handling logic to properly assign the subsecond component if
built with nanosecond support, or silently discard it otherwise.
Also, re-enable the time attribute for the kyua tests.