from a counter, which allowed 2 depths of nesting (any 2 hot key handlers)
to a bit-mask which disallows *any* hot key handler to be re-entrant, but
you can enter any of the *other* hot key handlers from any *other* hot
key handler.

Previously, for example, you could hit Ctrl-P to enter the private message
prompt and then Ctrl-P again to re-enter the private message prompt
(recusively) - confusingly, requiring 2 'Q'uits to get back to whatever
you were doing before you hit the first Ctrl-P. Been like this forever.
Now, while in the Ctrl-P (private message) handler, hitting Ctrl-P again
does nothing. But you can hit Ctrl-T, or Ctrl-K or any other global hot
key and it'll work but those in turn will also disallow recursion of their
own handlers.