Wrap posix_fadvise(2) and posix_fallocate(2)

This adds two new functions in `System.Posix.Unistd`

 - `fileAdvise` (aka `posix_fadvise(2)`), and
 - `fileAllocate` (aka `posix_fallocate(2)`)

This is based in part on #7 and has been heavily refactored from its
original patch submission by Ricardo Catalinas Jiménez.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>