Have iomux_update_events be a no-op if the events didn't change

src/iomux_impl.c

@@ -130,13 +130,21 @@ int iomux_unregister_fd(struct IOMux *mux, struct IOMuxFD *fd)
     return mux->unregister_fd_impl(mux, fd);
 }
 
-int iomux_update_events(struct IOMux *mux, struct IOMuxFD *fd)
+int iomux_update_events(struct IOMux *mux, struct IOMuxFD *fd, unsigned int what)
 {
+    int rc;
     assert(mux != NULL);
     assert(fd != NULL);
     assert(fd->callback != NULL);
     assert(fd->fd >= 0);
-    return mux->update_events_impl(mux, fd);
+    
+    if (fd->what != what) {
+        mux->update_events_impl(mux, fd);
+    } else {
+        rc = 0;
+    }
+
+    return rc;
 }
 
 struct UCoreClock *iomux_get_clock(struct IOMux *mux)