The auth provider setup is a bit over-engineered. Simplify it by
making a single MMAuthProvider object that may or may not use polkit,
depending on configure options. This object is also setup as a
singleton object using the helper MM_DEFINE_SINGLETON_GETTER().
We get rid of the MMAuthRequests; and we leave up to the caller and user of the
MMAuthProvider the handling of request cancellations through the provided
GCancellable.
Also made GIO-async-friendly methods.
While porting to GDBus, use opaque pointers. This allows us to include either a
DBusGMethodInvocation or a GDBusMethodInvocation in the 'context' pointer.
Once fully ported to GDBus, we can safely change it back to make the context be
a GDBusMethodInvocation.
When the support is complete, use --with-polkit to enable
PolicyKit support. It's not there yet, but this commit adds an
authorization provider framework which will be extended to allow
hooking into PolicyKit.
