Fix matching of incoming email addresses to the alternate_addresses

Fix matching of incoming email addresses to the alternate_addresses
field of a user -- this would match substrings, e.g. if the user has
discuss-support@example.com as an alternate email and an incoming mail
is addressed to support@example.com this would (wrongly) match.

Note: I *think* I've seen this discussed somewhere but couldn't find it,
neither in the tracker nor in recent discussions on the mailinglists.
So if someone remembers an issue which now should be closed, please
tell me :-)

git-svn-id: http://svn.roundup-tracker.org/svnroot/roundup/roundup/trunk@4644 57a73879-2fb5-44c3-a270-3262357dd7e2