zz: restore sandbox function labelling for undo, killring etc.

The maintenance of runtime labels that functions such as undo use
to determine whether they are being called consecutively was
accidentally removed with c908ad8.
This changeset restores the missing functionality, so undo and
other functions that rely on it work properly again.
* lib/zz/eval.lua (labelfn): New function, return a wrapper
function that when called manages labels correctly.
(Defun): Ensure new symbol value and __call metatable entries
call the label managing wrapper function.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>