More mime type tightening

[Christopher Faylor]

The old checks for mime types like text/html didn't deal with nested
messages.  I just changed my spam blocker to check for this, however, after
noticing some spam get through which should have been blocked.

That means that some email, containing html, which used to get through
will now be blocked.  I saw one such block in gcc-help just now.

I'm just alerting people to this fact.  There is no change in our policy
of not accepting html mail.  We're just better at detecting it now.

cgf