MINOR: protocol: add a new pair of rx_enable/rx_disable methods

These methods will be used to enable/disable rx at the receiver level so
that callers don't play with FDs directly anymore. All our protocols use
the generic ones from sock.c at the moment. For now they're not used.
diff --git a/src/proto_sockpair.c b/src/proto_sockpair.c
index b787f95..4a5701c 100644
--- a/src/proto_sockpair.c
+++ b/src/proto_sockpair.c
@@ -36,6 +36,7 @@
 #include <haproxy/listener.h>
 #include <haproxy/protocol.h>
 #include <haproxy/proto_sockpair.h>
+#include <haproxy/sock.h>
 #include <haproxy/time.h>
 #include <haproxy/tools.h>
 #include <haproxy/version.h>
@@ -67,6 +68,8 @@
 	.sock_prot = 0,
 	.add = sockpair_add_listener,
 	.listen = sockpair_bind_listener,
+	.rx_enable = sock_enable,
+	.rx_disable = sock_disable,
 	.accept = &listener_accept,
 	.connect = &sockpair_connect_server,
 	.receivers = LIST_HEAD_INIT(proto_sockpair.receivers),