When processing mtree(5) MANIFEST files, makefs(8) previously threw an
error if it encountered an entry whose "time" attribute contained 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.
PR: 194703
Submitted by: Mitchell Horne <mhorne063@gmail.com>
Differential Revision: https://reviews.freebsd.org/D19627
