os: Add facilities for client ID tracking.

An interface is provided for figuring out the PID and process name of
a client. Make some existing functionality from SELinux and IA
extensions available for general use.

Signed-off-by: Rami Ylimäki <rami.ylimaki@vincit.fi>
Reviewed-by: Tiago Vignatti <tiago.vignatti@nokia.com>