This does not actualy fit the project name "liblongpath", but most of the
surrounding code is identical. So for the time being I will tolerate
the fact that liblongpath contains a little side use, but should we want
to integrate more functionality, we should probably give the project a
more generic name.

Use of the interposer by environment variables

LD_PRELOAD=<path_to_lib>

* standard behaviour: map O_DIRECT to O_DSYNC for open() failing
  with EINVAL
* set CONVODIRECT=none to just ignore O_DIRECT