Support SOCK_DGRAM and SOCK_SEQPACKET unix domain sockets

Created by: laf0rge

This introduces a new testport parameter "socket_type", which can be set either to "STREAM" (default), "DGRAM" or "SEQPACKET" in order to determine the socket type of the underlying unix domain socket.

Please note that SOCK_SEQPACKET unix domain sockets are present on Linux but may not be present on other operating systems.