DKIM: if one of signatures are correct for dkim domain, then use it.

[BHYCHIK]

DKIM: if one of signatures are correct for dkim domain, then use it, not the first one.

[z3APA3A]

Currently first signature is always used. In situation where message has both valid and invalid DKIM signatures for the same domain (e.g. message is sent to mailing list or signed by forwarder) it may lead to the situation where valid DKIM signature is ignored and message is treated as unsigned. Valid DKIM signature should always be preffered.