MEDIUM: listeners: implement protocol level ->suspend/resume() calls

Now we have ->suspend() and ->resume() for listeners at the protocol
level. This means that it now becomes possible for a protocol to redefine
its own way to suspend and resume. The default functions are provided for
TCP, UDP and unix, and they are pass-through to the receiver equivalent
as it used to be till now. Nothing was defined for sockpair since it does
not need to suspend/resume during reloads, hence it will succeed.
diff --git a/src/proto_udp.c b/src/proto_udp.c
index 1c2477e..13545ad 100644
--- a/src/proto_udp.c
+++ b/src/proto_udp.c
@@ -61,6 +61,8 @@
 	.enable = udp_enable_listener,
 	.disable = udp_disable_listener,
 	.unbind = default_unbind_listener,
+	.suspend = default_suspend_listener,
+	.resume  = default_resume_listener,
 	.rx_enable = sock_enable,
 	.rx_disable = sock_disable,
 	.rx_unbind = sock_unbind,
@@ -85,6 +87,8 @@
 	.enable = udp_enable_listener,
 	.disable = udp_disable_listener,
 	.unbind = default_unbind_listener,
+	.suspend = default_suspend_listener,
+	.resume  = default_resume_listener,
 	.rx_enable = sock_enable,
 	.rx_disable = sock_disable,
 	.rx_unbind = sock_unbind,