Network-related giomodules for glib.

Glib-networking contains the implementations of certain GLib networking
features that cannot be implemented directly in GLib itself because of their
dependencies.

Currently it contains a GNUTLS-based implementation of GTlsBackend,
a libproxy-based implementation of GProxyResolver, and a GNOME GProxyResolver
that uses the proxy information from the GSettings schemas in
gsettings-desktop-schemas.