Wrap fsync(2) and fdatasync(2)

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

 - `fileSynchronise` (aka `fsync(2)`), and
 - `fileSynchroniseDataOnly` (aka `fdatasync(2)`)

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

This also bumps version to 2.7.1.0 as a minor version bump is now needed.

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