pkcheck: Support --process=pid,start-time,uid syntax too

pkcheck: Support --process=pid,start-time,uid syntax too

The uid is a new addition; this allows callers such as libvirt to
close a race condition in reading the uid of the process talking to
them.  They can read it via getsockopt(SO_PEERCRED) or equivalent,
rather than having pkcheck look at /proc later after the fact.

Programs which invoke pkcheck but need to know beforehand (i.e.  at
compile time) whether or not it supports passing the uid can
use:

pkcheck_supports_uid=$($PKG_CONFIG --variable pkcheck_supports_uid polkit-gobject-1)
test x$pkcheck_supports_uid = xyes