add module loading to PolicyKit

add module loading to PolicyKit

This paves the way for writing

 1. A module that tracks temporary (look in /var/run) and permanent (look
    in /var/lib) privilege grants
 2. A D-Bus service to authenticate a client to obtain to a privilege
    grant and then writing the grant in temporary or permanent storage

Also, this feature lets people very easily lock down the system; just
edit /etc/PolicyKit/PolicyKit.conf; add pam-module-deny-all / -allow-all
stanzas with various privilege=<regexp> and user=<username> options.