REORG/MEDIUM: move the default accept function from sockstream to protocols.c

The previous sockstream_accept() function uses nothing from sockstream, and
is totally irrelevant to stream interfaces. Move this to the protocols.c
file which handles listeners and protocols, and call it listener_accept().

It now makes much more sense that the code dealing with listen() also handles
accept() and passes it to upper layers.
diff --git a/src/proto_uxst.c b/src/proto_uxst.c
index 2140ae6..1a808ea 100644
--- a/src/proto_uxst.c
+++ b/src/proto_uxst.c
@@ -55,7 +55,7 @@
 	.sock_family = AF_UNIX,
 	.sock_addrlen = sizeof(struct sockaddr_un),
 	.l3_addrlen = sizeof(((struct sockaddr_un*)0)->sun_path),/* path len */
-	.accept = &stream_sock_accept,
+	.accept = &listener_accept,
 	.bind = uxst_bind_listener,
 	.bind_all = uxst_bind_listeners,
 	.unbind_all = uxst_unbind_listeners,