mailing list updates

The XFree86 mailing lists have been modified on an experimental
basis to require subscription for posting.  Perhaps this will be
enough to eliminate the < 0.1% of spam that our existing filtering
does not already catch.  The existing filtering (spam assassin plus
a home-grown filter) will remain in place.

If this does not prove effective, or if the list admin burden
increases signficantly (i.e., posters do not subscribe), then the
behaviour may be reverted.

David