exim: act correctly on dnsbl+authentication

When an (possibly) authenticated user tried to send mail from an address
listed on any DNSBL, he was rejected without any chance to authenticate,
thus proving he is not a spammer.

This commit rearranges the order of checks so that authentication is
possible even from DNSBL'd IP.
Merge pull request #178 from exaexa/dnsbl-vs-login-fix

Exim ACL fix: act correctly on DNSBL+authentication