provide convenience functions for auth checking and port helpers to use them

provide convenience functions for auth checking and port helpers to use them

Basically, checking auths with polkit is now a one-liner:

    if (polkit_check_auth (getpid (), "com.acme.some-action", NULL) == 0) {
        fprintf (stderr, "Not authorized; go away\n");
        exit (1);
    }

This can be used for making a lot of the legacy UNIX tools PolicyKit
aware. For example, vixie-cron could make crontab(1) (a setuid
program) check whether the calling user is authorized for the action

 org.isc.vixie-cron.edit-own-crontab

This is a nice way to provide least privilege and still put the system
administrator in control via polkit-auth(1), polkit-action(1) and the
GTK+ "Manage Authorizations" utility:

 http://people.redhat.com/davidz/polkitg-auth-1.png
 http://people.redhat.com/davidz/polkitg-auth-2.png
 http://people.redhat.com/davidz/polkitg-auth-3.png