Support NetBSD

NetBSD uses different sizes of the members `filter`, `flags` of the structure `kevent`. It also uses different numbering of the `EVFILT_` flags.

I've compared the `event.h` header files of FreeBSD, NetBSD and OpenBSD and changed the files `lib/rb-kqueue/native.rb` and `lib/rb-kqueue/native/flags.rb` to match what I've found there regarding the `kevent` structure and the `EVFILT_` flags.

c.f.
```
http://svnweb.freebsd.org/base/head/sys/sys/event.h?revision=HEAD&view=co
http://cvsweb.netbsd.org/bsdweb.cgi/~checkout~/src/sys/sys/event.h?rev=HEAD&content-type=text/plain
http://www.openbsd.org/cgi-bin/cvsweb/src/sys/sys/event.h?rev=HEAD;content-type=text%2Fplain
```