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/gresolver.c

@@ -76,7 +76,8 @@ struct _GResolverPrivate {
  * The object that handles DNS resolution. Use g_resolver_get_default()
  * to get the default resolver.
  */
-G_DEFINE_TYPE (GResolver, g_resolver, G_TYPE_OBJECT)
+G_DEFINE_TYPE_WITH_CODE (GResolver, g_resolver, G_TYPE_OBJECT,
+			 g_networking_init ();)
 
 static GList *
 srv_records_to_targets (GList *records)
@@ -153,9 +154,6 @@ g_resolver_class_init (GResolverClass *resolver_class)
 
   g_type_class_add_private (resolver_class, sizeof (GResolverPrivate));
 
-  /* Make sure _g_networking_init() has been called */
-  g_type_ensure (G_TYPE_INET_ADDRESS);
-
   /**
    * GResolver::reload:
    * @resolver: a #GResolver