glib/win32: introduce private g_win32_reopen_noninherited()

Used in following commits, including in some GIO experiments, so make it
a private API.

For now, this implementation is similar to the glib/gspawn-win32.c one,
with mroe error checking and better on error behaviour. A following
patch will also fix the case of duplicating sockets.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

glib/glib-private.c

@@ -55,6 +55,7 @@ glib__private__ (void)
     g_win32_readlink_utf8,
     g_win32_fstat,
     g_win32_find_helper_executable_path,
+    g_win32_reopen_noninherited,
 #endif
   };