Traditionally the functions in os are thin wrappers around OS functions, this patch changes that for os.fsync.


Two nits wrt. the patch itself:
1) The behaviour on OSX should be documented in the stdlib reference,
   that currently says that os.fsync will call the C function fsync

2) The comment in the __APPLE__ case can be clearer, and should 
   explain the issue (as short as possible).


I'm -1 on merging this patch though, as the fsync function on OSX behaves just like the one on Linux, the fnctl option adds a stronger guarantee in that it forces a flush of the buffers on the storage device as well. The manpage for fsync on a linux host says:

   In case the hard disk has write cache enabled, the data may not really be on permanent storage when fsync/fdatasync return.

Adding the F_FULLSYNC option to os.fsync would be fine though (if it isn't already implemented)

The linux