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/gunixconnection.c

@@ -39,16 +39,12 @@
 
 #include <gio/gsocketcontrolmessage.h>
 #include <gio/gunixfdmessage.h>
+#include <gio/gnetworking.h>
 #include <gio/gsocket.h>
-#include <unistd.h>
 
-#ifdef __linux__
-/* for getsockopt() and setsockopt() */
-#include <sys/types.h>          /* See NOTES */
-#include <sys/socket.h>
 #include <errno.h>
 #include <string.h>
-#endif
+#include <unistd.h>
 
 
 G_DEFINE_TYPE_WITH_CODE (GUnixConnection, g_unix_connection,