rewriting y-or-n-p to use read-char-choice

[Ted Zlatanov]

Stefan Monnier asked me to look at rewriting `y-or-n-p' to use
`read-char-choice'.  This looked easy at first but I realized it's not
such a good idea because:

- `y-or-n-p' uses `query-replace-map' which could be modified
  externally.  So it would have to gather the key choices from the key
  map, and even that's not possible because it takes the default, so any
  character can potentially be meaningful.

- `read-char-choice' has special code for keyboard macros which I think
  will break `y-or-n-p'; `y-or-n-p' OTOH calls `discard-input'

- the amount of duplicated code is not much at all

So I think a better way would be to write a
`read-char-choice-from-keymap' function which can be passed
`query-replace-map' or something else AND a prompt and a list of
acceptable return values.

This function then calls `lookup-key' on the keyboard input and returns
a symbolic value.  This function could also build the a/b/c part of the
prompt dynamically based on the acceptable key choices.  But if
`y-or-n-p' is the only consumer for this functionality, it seems
pointless to add it.

Ted