From: James Morris <jmorris@redhat.com>

This patch adds a new option for Unix sockets, SO_PEERSEC, and an
associated LSM hook, getpeersec.  The SELinux handler is also included.

The purpose of this is to allow applications to obtain each others security
credentials, analagously to the existing SO_PEERCRED option.

Examples of use are Security Enhanced D-BUS and Security Enhanced X.

This patch was previously approved in principle by David, and has been
updated with feedback from Chris Wright and extended to cover all
architectures.