Add support for more OpenFileFlags and refactor 'openFd' (PR #59)

* Add support for `O_NOFOLLOW`, `O_CLOEXEC`, `O_DIRECTORY` and `O_SYNC`
   (#6, #57)

* Refactor API of `openFd` removing `Maybe FileMode` argument,
   which now must be passed as part of `OpenFileFlags`
   (e.g. `defaultFileFlags { creat = Just mode }`)  (#58)

Closes #59