make check_perms ensure aliases.db is group-writeable

checkperms in the Postfix module doesn't ensure that
both aliases and aliases.db are group writable. The
attached patch against CVS (20051211) fixes this by
copying the check for aliases to also check aliases.db.
 It also patches mailman-install.tex to make it clear
that both files should be group writable.

See Tracker 1204386[1] and Mailman FAQ 6.9[2] for
reports on this problem.

[1]
https://sourceforge.net/tracker/?func=detail&atid=100103&aid=1204386&group_
id=103
[2]
http://www.python.org/cgi-bin/faqw-mm.py?req=show&file=faq06.009.htp

[http://sourceforge.net/tracker/index.php?func=detail&aid=1378270&group_id=103&atid=100103]