MEDIUM: connections: Change struct wait_list to wait_event.

When subscribing, we don't need to provide a list element, only the h2 mux
needs it. So instead, Add a list element to struct h2s, and use it when a
list is needed.
This forces us to use the unsubscribe method, since we can't just unsubscribe
by using LIST_DEL anymore.
This patch is larger than it should be because it includes some renaming.
diff --git a/include/types/checks.h b/include/types/checks.h
index d148e3b..e0a4bd6 100644
--- a/include/types/checks.h
+++ b/include/types/checks.h
@@ -184,7 +184,7 @@
 	char **envp;				/* the environment to use if running a process-based check */
 	struct pid_list *curpid;		/* entry in pid_list used for current process-based test, or -1 if not in test */
 	struct sockaddr_storage addr;   	/* the address to check */
-	struct wait_list wait_list;            /* Waiting for I/O events */
+	struct wait_event wait_list;            /* Waiting for I/O events */
 	char *sni;				/* Server name */
 };