Fix the file position handling of the read and write calls

Since the interface is modelled after the libc calls we should try to match
their behaviour to avoid unpleasant surprises:
* read / write (sync and async flavours) update the file position
* pread / pwrite (sync and async flavours) do not update the file position
.

Signed-off-by: Arne Redlich <arne.redlich@googlemail.com>