nanobsd: Use mtree -C to produce the metalog

Prefer an mtree -C output, which is guaranteed to be mtree-compatible.
Add "gname", "uname", and "tags" to the default keyword set, while
removing "size" and "time", the latter being set on kernel file entries
and taking precedence over makefs -T (when paired with -F).

As a side effect, this produces a cleaner file with sorted keywords.

Note that passing "-u" to sort in order to pipe to mtree is no longer
necessary, but we'll do it out of habit.

Reviewed by: imp
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D54854