pkg-config: add a couple of additional variables to .pc

pkg-config: add a couple of additional variables to .pc

We already export the session services dir in the .pc file, add the
system services dir too, as well as the interfaces dir, so that packages
can drop their files into the right place.

Eventually we need to move the system-services dir to a place that is
relocatable to /lib/dbus, and this change makes the location queriable
for other projects, as a first step towards that goal.