pop-up-windows value to force a new window

["Drew Adams" <drew.adams@oracle.com>]

I don't see a good way to force `display-buffer' and related functions to
always use another window, even when there is more than one window displayed
currently. Is there some way (besides defining `display-buffer-function' in
such a way that it does this)?

What about letting a special non-nil value of `pop-up-windows', say
`new-window', cause this to happen? That is, if `pop-up-windows' =
`new-window', then `display-buffer', `pop-to-buffer', etc. would always use
a new window; they would never reuse an existing window.

This would let Lisp code, for example, bind to that value, and be sure that
`display-buffer' etc. will always create a new window. Subsequently deleting
that window will not leave fewer windows than before the display. IOW, this
would let you use `display-buffer' to always pop up an extra window.

This should have no effect on existing code (unless there is some code that
already uses that special value).