Add multi-port wrappers for UDPMux and TCPMux

These wrappers allow a caller to provide UDPMux and TCPMux instances
to the ICE agent that represent multiple open ports. This can be
desirable in what would otherwise be single-port deployments, as it
increases the chance that one of the fixed ports will not be blocked
by a users firewall.

errors.go

@@ -139,4 +139,6 @@ var (
 	errSendSTUNPacket                = errors.New("failed to send STUN packet")
 	errXORMappedAddrTimeout          = errors.New("timeout while waiting for XORMappedAddr")
 	errNotImplemented                = errors.New("not implemented yet")
+	errNoUDPMuxAvailable             = errors.New("no UDP mux is available")
+	errNoTCPMuxAvailable             = errors.New("no TCP mux is available")
 )