lib/selection{,_iter}.c: allow switching back to UDP

lib/selection{,_iter}.c: allow switching back to UDP

Switching to TCP instead of querying very slow servers over UDP has had
unwanted side effect – we would sometimes get stuck with a server
permanently switched to TCP. And if the server happens to not reply over
TCP we were in trouble.

Therefore after we TCP connect fails or timeouts we provide one last
chance for the server over UDP. This will not prevent the next request
to try TCP again on this server again, but we don't care because
DNS MUST ******* work over TCP.