MINOR: quic: define QUIC flag on listener

Mark QUIC listeners with the flag LI_F_QUIC_LISTENER. It is set by the
proto-quic layer on the add listener callback. This allows to override
more clearly the accept callback on quic_session_accept.
diff --git a/src/cfgparse.c b/src/cfgparse.c
index 6ce2f52..3dfaf65 100644
--- a/src/cfgparse.c
+++ b/src/cfgparse.c
@@ -66,6 +66,7 @@
 #include <haproxy/log.h>
 #include <haproxy/mailers.h>
 #include <haproxy/namespace.h>
+#include <haproxy/quic_sock.h>
 #include <haproxy/obj_type-t.h>
 #include <haproxy/peers-t.h>
 #include <haproxy/peers.h>
@@ -3901,11 +3902,13 @@
 			if (!listener->maxaccept)
 				listener->maxaccept = global.tune.maxaccept ? global.tune.maxaccept : MAX_ACCEPT;
 
-			/* listener->accept may already be initialized by some
-			 * protocols so do not overwrite it in this case.
-			 */
-			if (!listener->accept)
-				listener->accept = session_accept_fd;
+			/* listener accept callback */
+			listener->accept = session_accept_fd;
+#ifdef USE_QUIC
+			/* override the accept callback for QUIC listeners. */
+			if (listener->flags & LI_F_QUIC_LISTENER)
+				listener->accept = quic_session_accept;
+#endif
 
 			listener->analysers |= curproxy->fe_req_ana;
 			listener->default_target = curproxy->default_target;
diff --git a/src/proto_quic.c b/src/proto_quic.c
index 2834845..6fdb3f7 100644
--- a/src/proto_quic.c
+++ b/src/proto_quic.c
@@ -518,8 +518,7 @@
  */
 static void quic_add_listener(struct protocol *proto, struct listener *listener)
 {
-	/* custom accept callback for QUIC */
-	listener->accept = quic_session_accept;
+	listener->flags |= LI_F_QUIC_LISTENER;
 
 	MT_LIST_INIT(&listener->rx.pkts);
 	listener->rx.odcids = EB_ROOT_UNIQUE;