core: reset processor affinity when starting

Open-MPI 1.8.1 binds processes to cores (default
behavior). Therefore, any thread started after
starting MPI will binded to the same core.

This change is a workaround which basically resets
the affinity (on Linux) so that threads are not
binded to cores.

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
structures: use static keyword for private ring functions

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>