os.notify: implement the kqueue backend for notify.FdNotifier (#19057)

vlib/os/notify/kqueue.h

@@ -0,0 +1,12 @@
+#ifndef __KQUEUE_H
+#define __KQUEUE_H
+
+#include <sys/event.h>
+
+// Due to the renaming of 'struct kevent' and function 'kevent',
+// they are wrapped here to avoid conflicts.
+int __kevent__(int handle, const struct kevent* changelist, int nchanges, struct kevent* eventlist, int nevents, const struct timespec* timeout) {
+	return kevent(handle, changelist, nchanges, eventlist, nevents, timeout);
+}
+
+#endif