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 GnuTLS and OpenSSL-based implementations of GTlsBackend, 
a libproxy-based implementation of GProxyResolver, GLibproxyResolver,
and a GNOME GProxyResolver that uses the proxy information from the GSettings
schemas in gsettings-desktop-schemas.

File bugs against
https://gitlab.gnome.org/GNOME/glib-networking/issues/new
