[luatex] multiple callbacks

[Karl Berry]

Excuse me for coming in late, and with such an obvious comment, but how
about handling such callback in lua a la hooks in Emacs?  The hook
author can decide whether they want to ask that it be at the front of
the list, the back of the list, or (default) don't care.  Of course
there are no guarantees due to multiple packages requesting the same
thing, but at least the basic options would be provided.