This change will futureproof libpolkit for extensions; e.g. if there's
an OS upgrade where

 a) the PolicyKit package is upgraded to a version where support for a
    new tag <allow_foo> is added; and

 b) another package, using PolicyKit, is upgraded dropping a .policy
    file using the new <allow_foo> tag; then

existing running processes using libpolkit will not fail. They will,
however, not honor the new tags until the daemon process itself is
restarted using e.g. condrestart.

We also log to the system logger whenever we encouter unknown tags.