Add g_unix_open_pipe_internal () for pipes with the close-on-exec flag

Based on the existing g_unix_open_pipe () but for internal use where
returning a raw errno is needed, not a GError.

docs/reference/glib/meson.build

@@ -38,6 +38,7 @@ if get_option('gtk_doc')
     'gutilsprivate.h',
     'gvalgrind.h',
     'dirent.h',
+    'glib-unixprivate.h',
     'glib-visibility.h',
     'gmodule-visibility.h',
   ]