Sort pkgbase mtree metadata, for reproducible builds

Packaged base packages are created by running the stageworld and
stagekernel targets with -DNO_ROOT, and converting the resulting mtree
file into a set of pkg plists. If stage* is run with multiple processes
the order of entries in the mtree file may be nondeterministic, and the
resulting package tbz also had nondeterministic file ordering.

The mtree file generated by -DNO_ROOT builds consists of one line per
file, with the filename starting in the first column, so is easily
sorted. There's one exception: the first line of the mtree file is a
comment, but the # character sorts before the filenames anyhow and needs
no special treatment.

PR: 223673
Reviewed by: bapt, gjb
Sponsored by: The Linux Foundation, Core Infrastructure Initiative
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D13103