bug#8857: display-buffer attempt to pop-up frame in batch mode causes "Unknown terminal type" error

[David Engster]

martin rudalics writes:
>> However, this
>> effectively means that there really is no default value for
>> pop-up-frames,
>
> ... well it's 'unset ...

Yes. But I think you understand my problem: 'unset doesn't say whether
you should pop up a new frame by default or not. It's pretty much what
Drew says in #8865, and I tend to agree with him on this issue. I think
this special handling of 'unset is cumbersome, and it should simply be a
boolean variable with a default of nil or t.

Anyway, I don't want to bother you with two threads on the same issue,
and the original problem has been solved, so I'm closing this bug and
will maybe follow-up on #8865, but I think pretty much everything has
been said regarding this issue.

-David