conncheck: avoid transport association mismatch

In some rare cases, the same address and port number may match two remotes candidates, a tcp and an udp one, and lead to buggy pair construction with incompatible transport. This supplementary check prevents this problem. The matching test is not aimed to be exhaustive but just a way to discard obviously broken associations, and fallback to accept everything else (because socket type has a great diversity, with socket types based on other sockets types).

It should fix #81, where such bogus transport association has been reported (tcp-pass:udp).