base.m4: More subtle handling of HELO greetings.

Rather than rejecting incorrect HELO greetings, we make two adjustments:

  * There's now an auxiliary list, `helo.conf', of manually provided
    exceptions, for well-known and basically honest hosts which are just
    misconfigured.

  * A failure to provide a correct HELO greeting now results in a
    BADHELO warning header rather than an immediate rejection.
    SpamAssassin has been configured to notice these headers and assign
    points for them, because they do seem to be a good indicator of
    spamminess.