1. support lua 5.1 to lua 5.4 by substitution of lua_objlen with lua_rawlen
2. implemented kqueue watch monitor

As discussed in previous threads, kqueue is really a crippled API comparing to inotify, and has a few problems with this task. Here is how I workaround its limitations.

1. Each file need to be opened to be watched. Files would be opened with flags to only get events (with O_EVTONLY) if possible, and is closed upon any event
2. kqueue could not track move properly like inotify. Here I use the upper directory write event to emit a 'Modify' dir delay, and have it interpreted as 'something changed in this dir, but we don't know what exactly', fortunately rsync is powerful enough to handle this.
3. kqueue got closed on fork. In freebsd this is fixed by using rfork instead, and in macos a func is called after the fork to recreate the queue.

I tested my impl on FreeBSD & MacOS, and it works fine. This also removes the limitation that it depends on super user & internal xnu interface, which is my initial motivation (to use this project with mbp). Personally I take this as a chance to practice using kqueue as well, so please help me if I messed up sth bleh 😺 .