Add gnetworking.h

Install a public "gnetworking.h" header that can be used to include
the relevant OS-dependent networking headers. This does not really
abstract away unix-vs-windows however; error codes, in particular,
are incompatible.

gnetworkingprivate.h now contains just a few internal URI-related
functions

Also add a g_networking_init() function to gnetworking.h, which can be
used to explicitly initialize OS-level networking, rather than having
that happen as a side-effect of registering GInetAddress.

https://bugzilla.gnome.org/show_bug.cgi?id=623187

gio/.gitignore

@@ -10,6 +10,7 @@ gio-querymodules
 gioenumtypes.[ch]
 glib-compile-resources
 glib-compile-schemas
+gnetworking.h
 gresource
 gschema-compile
 gsettings