Added Pfadvise and constant advise values. Poorly tested.

Hello! Thanks for luaposix. :)
I've added posix_fadvise and the constant "advise" ints that go with it. A very cursory check shows it seeming to work; at least, it returns 1 and doesn't crash the interpreter.
I won't know if it really works well until I get my hands on the equipment I need it for and check if it bypasses caching.
best,
Cathal