As it turns out, we have examples of internal functions called
type_name_get_private() in the wild (especially among older libraries),
so we need to use a name for the per-instance private data getter
function that hopefully won't conflict with anything.
Add GSimpleProxyResolver, for letting people do static proxy
resolution, and to use as a base class for other resolvers (such as
GProxyResolverGnome).
https://bugzilla.gnome.org/show_bug.cgi?id=691105