MEDIUM: stream_interface: derive the socket operations from the target

Instead of hard-coding sock_raw in connect_server(), we set this socket
operation at config parsing time. Right now, only servers and peers have
it. Proxies are still hard-coded as sock_raw. This will be needed for
future work on SSL which requires a different socket layer.
diff --git a/include/types/server.h b/include/types/server.h
index 31f036d..aa2c4f8 100644
--- a/include/types/server.h
+++ b/include/types/server.h
@@ -149,6 +149,8 @@
 	int bind_hdr_occ;			/* occurrence number of header above: >0 = from first, <0 = from end, 0=disabled */
 #endif
 	struct protocol *proto;	                /* server address protocol */
+	struct sock_ops *sock;                  /* server socket operations */
+	void *sock_init_arg;                    /* socket operations's opaque init argument if needed */
 	unsigned down_time;			/* total time the server was down */
 	time_t last_change;			/* last time, when the state was changed */
 	struct timeval check_start;		/* last health check start time */