pw: Add a metalog output mode

[markj]

Summary

When creating OS images as a non-root user, it may be useful to
pre-create users in the staged tree. The useradd operation adds files
to the new user's home directory, copied from the skeleton directory
(/usr/share/skel), which makes it inconvient for use in this scenario.

To cover this gap, this change adds a new -M <metalog> option to pw's
useradd operation, causing pw to add mtree entries for newly added
files.

[emaste]

usr.sbin/pw/pw.c

260–261

maybe worth doing this bugfix separately so we can MFC it in advance of other changes here?

usr.sbin/pw/pw_utils.c

97

This _ convention seems a bit unusual

[bapt]

usr.sbin/pw/pw_utils.c

124

I wonder if it won't be smart to move the metalog_emit to libutil later so it can be used by other tools

[markj]

usr.sbin/pw/pw_utils.c

97

Ok, I will rename it to metalog_emit_record().

116

I'm not handling file flags here. I don't think pw sets any, but we should handle that anyway.

124

Maybe, yeah. It's a fairly simple routine though, and different consumers have different requirements (e.g., uname/gname vs. uid/gid, some consumers want to flock the metalog, ...).