gmain: Add g_steal_fd() to API

This is basically glnx_steal_fd() from libglnx. We already had two
private implementations of it in GLib.

Signed-off-by: Simon McVittie <smcv@collabora.com>

glib/gmain.h

@@ -781,6 +781,15 @@ void     g_main_context_invoke      (GMainContext   *context,
                                      GSourceFunc     function,
                                      gpointer        data);
 
+GLIB_AVAILABLE_STATIC_INLINE_IN_2_70
+static inline int
+g_steal_fd (int *fd_ptr)
+{
+  int fd = *fd_ptr;
+  *fd_ptr = -1;
+  return fd;
+}
+
 /* Hook for GClosure / GSource integration. Don't touch */
 GLIB_VAR GSourceFuncs g_timeout_funcs;
 GLIB_VAR GSourceFuncs g_child_watch_funcs;