postprocess: Write preserved groups to *both* /etc/group and /usr/lib/gr...

[cgwalters]

...oup

Otherwise, upgraded systems which have modified /etc/group (by e.g.
adding a human user), will actually see the group drop out with bad
consequences.

It's harmless to have it in both, /etc will override /usr.

Fixes #67

[mbarnes]

The new logic could be a tad clearer with:

if (id == 0)
deststream = etcdest_stream;
else
deststream = usrdest_stream;

Also I'd suggest a comment before the assertion, since it took me a minute to figure out what that was actually asserting. Maybe something like:

/* Don't write the root entry to /etc twice. */
g_assert (deststream == usrdest_stream);

[cgwalters]

Made both those changes, thanks!