This paves the way for getting rid of a glib dependency; when and if
that happens is to be determined; right now it just doesn't make a lot
of sense to reimplement GKeyFile, GSList, GHashTable, g_spawn_sync and
other useful routines. But it might make sense if we want to get the
message bus daemon to link with libpolkit so you e.g. can say

 <policy polkit="acme-frobnicate">
   <allow send_interface="com.acme.Frobnicator"
 </policy>

to allow a caller on the system message bus to access that interface
if, and only if, he can do the "acme-frobnicate" action according to
PolicyKit.