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_tcp.c b/src/proto_tcp.c
index 478bff5..0c389d8 100644
--- a/src/proto_tcp.c
+++ b/src/proto_tcp.c
@@ -69,7 +69,7 @@
 	.sock_family = AF_INET,
 	.sock_addrlen = sizeof(struct sockaddr_in),
 	.l3_addrlen = 32/8,
-	.accept = &stream_sock_accept,
+	.accept = &listener_accept,
 	.connect = tcp_connect_server,
 	.bind = tcp_bind_listener,
 	.bind_all = tcp_bind_listeners,
@@ -88,7 +88,7 @@
 	.sock_family = AF_INET6,
 	.sock_addrlen = sizeof(struct sockaddr_in6),
 	.l3_addrlen = 128/8,
-	.accept = &stream_sock_accept,
+	.accept = &listener_accept,
 	.connect = tcp_connect_server,
 	.bind = tcp_bind_listener,
 	.bind_all = tcp_bind_listeners,