MINOR: protocol: add a new function accept_conn()

This per-protocol function will be used to accept an incoming
connection and return it as a struct connection*. As such the protocol
stack's internal representation of a connection will not need to be
handled by the listener code.
diff --git a/include/haproxy/protocol-t.h b/include/haproxy/protocol-t.h
index f2aa4af..499609d 100644
--- a/include/haproxy/protocol-t.h
+++ b/include/haproxy/protocol-t.h
@@ -94,6 +94,7 @@
 	void (*unbind)(struct listener *l);             /* unbind the listener and possibly its receiver */
 	int (*suspend)(struct listener *l);             /* try to suspend the listener */
 	int (*resume)(struct listener *l);              /* try to resume a suspended listener */
+	struct connection *(*accept_conn)(struct listener *l, int *status); /* accept a new connection */
 
 	/* functions acting on the receiver */
 	void (*rx_enable)(struct receiver *rx);         /* enable receiving on the receiver */