compose: Migrate content of /etc/{passwd,group} to /usr/lib more sanely

I had an epiphany today while working on
https://bugzilla.redhat.com/show_bug.cgi?id=1098304 - I realized that
I can just do an install, and then copy over everything except the
root entries from /etc/passwd into /usr/lib/passwd.

No need for a patched shadow-utils.  No need to modify the
/etc/nsswitch.conf before doing the install root.  It totally works.
I have no idea why I originally overcomplicated this.

The thing that sucks a bit about this code is that I have to drop to
the FILE * APIs so that I can use the glibc APIs for processing
group/shadow.

Also, the way I deduplicated the code paths for processing
passwd/group is crappy, but I think it's better than duplicating them
(as systemd-sysusers does).

The good: We don't need a two-step RPM transaction, we don't need
          a patch for shadow-utils, it's just saner
The bad: Code is not the most beautiful?  Not really bad.
The ugly: I didn't think of this in the first place and spent
          months beating my head against the wall of shadow-utils...