MFC r345281: makefs: Fix "time" mtree attribute handling

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: mhorne