base.m4: Be slacker about DH lengths.

Reduce the minimum size for general outbound SMTP to about 512 bits,
because (a) any DH is better than none, and (b) Exim will defer rather
than switching to plaintext if the receiving SMTP advertises STARTTLS.

Also introduce new transports with lower limits, and be consistent about
actually undershooting the advertised limit by four bits.