driver_test: Optional support for using UDP socket

driver_test: Optional support for using UDP socket

driver_test can now be used either over UNIX domain socket or UDP
socket. This makes it possible to run the test over network and makes it
easier to port driver_test to Windows.

hostapd configuration: test_socket=UDP:<listen port>
wpa_supplicant configuration: driver_param=test_udp=<dst IP addr>:<port>