sa,unixsock: add unix domain socket support

[sreimers]

Usage example:

	struct sa srv;
	re_sock_t fd;

	err = sa_set_str(&srv, "unix:/run/http.sock", 0);
	if (err)
		return err;

	err = unixsock_listen_fd(&fd, &srv);
	if (err)
		return err;

	re_printf("listen: %j\n", &srv);

	err = http_listen_fd(sock, fd, http_req_handler, mix);

[alfredh]

perhaps also update the list of modules:

https://github.com/baresip/re#modules

[alfredh]

what if the string is 5 bytes ?

[sreimers]

Since the provided addr string should be always zero terminated, sun_path is empty in this case too.

[alfredh]

I think this looks good now.

Please go ahead and merge, if you want