CLEANUP: protocol: remove the now unused <handler> field of proto_fam->bind()

We don't need to specify the handler anymore since it's set in the
receiver. Let's remove this argument from the function and clean up
the remains of code that were still setting it.
diff --git a/src/sock_unix.c b/src/sock_unix.c
index 2be8d6b..45631ac 100644
--- a/src/sock_unix.c
+++ b/src/sock_unix.c
@@ -118,12 +118,12 @@
 	return 0;
 }
 
-/* Binds receiver <rx>, and assigns <handler> and rx-> as the callback and
- * context, respectively, with <tm> as the thread mask. Returns and error code
- * made of ERR_* bits on failure or ERR_NONE on success. On failure, an error
- * message may be passed into <errmsg>.
+/* Binds receiver <rx>, and assigns rx->iocb and rx->owner as the callback and
+ * context, respectively, with ->bind_thread as the thread mask. Returns an
+ * error code made of ERR_* bits on failure or ERR_NONE on success. On failure,
+ * an error message may be passed into <errmsg>.
  */
-int sock_unix_bind_receiver(struct receiver *rx, void (*handler)(int fd), char **errmsg)
+int sock_unix_bind_receiver(struct receiver *rx, char **errmsg)
 {
 	char tempname[MAXPATHLEN];
 	char backname[MAXPATHLEN];
@@ -285,7 +285,7 @@
 	rx->fd = fd;
 	rx->flags |= RX_F_BOUND;
 
-	fd_insert(fd, rx->owner, handler, thread_mask(rx->settings->bind_thread) & all_threads_mask);
+	fd_insert(fd, rx->owner, rx->iocb, thread_mask(rx->settings->bind_thread) & all_threads_mask);
 
 	/* for now, all regularly bound TCP listeners are exportable */
 	if (!(rx->flags & RX_F_INHERITED))