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/gdbusauth.c

@@ -39,8 +39,7 @@
 #include "gdataoutputstream.h"
 
 #ifdef G_OS_UNIX
-#include <sys/types.h>
-#include <sys/socket.h>
+#include "gnetworking.h"
 #include "gunixconnection.h"
 #include "gunixcredentialsmessage.h"
 #endif