mmap: expose MAP_ANON

Hello!

I'm highly interested in using your module in my project [jit.js][0], however it doesn't seem to support `MAP_ANON` which I really need there since I'm not working with `fd`s.

Could you please consider pulling this changes and doing a npm release?

Cheers,
Fedor.

[0]: https://github.com/indutny/jit.js