tests: relax "Connection refused" match We already had the match relaxed on Windows, but on Google Compute Engine VMs I'm seeing "Network is unreachable" instead of "Connection refused". At this point, just give up and make sure we get an error back.

commandserver: backport handling of forking server from chgserver This is common between chg and vanilla forking server, so move it to commandserver and unify handle(). It would be debatable whether we really need gc.collect() or not, but that is beyond the scope of this series. Maybe we can remove gc.collect() once all resource deallocations are switched to context manager.