Add support for using 3-arg pkcheck syntax for process (CVE-2013-4311)

Add support for using 3-arg pkcheck syntax for process (CVE-2013-4311)

With the existing pkcheck (pid, start time) tuple for identifying
the process, there is a race condition, where a process can make
a libvirt RPC call and in another thread exec a setuid application,
causing it to change to effective UID 0. This in turn causes polkit
to do its permission check based on the wrong UID.

To address this, libvirt must get the UID the caller had at time
of connect() (from SO_PEERCRED) and pass a (pid, start time, uid)
triple to the pkcheck program.

This fix requires that libvirt is re-built against a version of
polkit that has the fix for its CVE-2013-4288, so that libvirt
can see 'pkg-config --variable pkcheck_supports_uid polkit-gobject-1'

Signed-off-by: Colin Walters <walters@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>