Alternative to SMART_ADDRESS_MATCH

This patch adds an extra configuration parameter,
COLLAPSIBLE_DOMAINS, containing a list of domains
within which all subdomains are considered equivalent,
for the purposes of verifying whether a post comes
from a list member (for lists where only members can
post).

For example, if COLLAPSIBLE_DOMAINS=['cam.ac.uk',
'ox.ac.uk'] and xxx@mrao.cam.ac.uk is subscribed to a
members-posting-only list, then messages from
xxx@hermes.cam.ac.uk or xxx@cam.ac.uk would not be
held.

This is intended to be an alternative to using
SMART_ADDRESS_MATCH.

Advantages over using SMART_ADDRESS_MATCH are:

- COLLAPSIBLE_DOMAINS only applies to domains you
know use common usernames across all sub-systems
(because you've explicitly listed them in mm_cfg.py
- SMART_ADDRESS_MATCH only works if the more general
form of the address is subscribed to the list and a
more
specific one is posted from, not the other way round
(e.g. if xxx@cam.ac.uk is subscribed, then posts from
xxx@hermes.cam.ac.uk *will* be held if you are just
using SMART_ADDRESS_MATCH. COLLAPSIBLE_DOMAINS works
both ways round.

Note that COLLAPSIBLE_DOMAINS only applies to verifying
whether a potential posting comes from a list member;
it doesn't attempt to match up email addresses when
people try to edit their subscription options on the
web or by email.

The uploaded file is a patch against 2.0.8 source.
Actually it's a patch against (2.0.8 with a few other
patches), but I've checked and the patch does correctly
apply against virgin 2.0.8 source.

If you apply this patch to the installation rather than
the source, you'll need to add the COLLAPSIBLE_DOMAINS
entry to your mm_cfg.py -- see comments in the patched
Defaults.py.in