bug#34516: Multi-monitor frame sets

[Robert Pluim]

>>>>> On Tue, 19 Mar 2019 23:33:30 +0200, Juri Linkov <juri@linkov.net> said:

    >> I can run the same test tomorrow when Iʼm by my external
    >> screens again. Only the first line here is remotely
    >> intelligible, none of them are displayed by macOS as far as I
    >> know.

    Juri> Did you have a chance to run the test?

I did, but I forgot to send the results:

--begin--
q8-@X,E
JC9H65BD0ZLS
DELL P2714H

q8-@X,E
JC9H65BD1BHS
DELL P2714H
--end--

Weʼd need to extract that 'DELL' bit, and check if we have other
monitors with the same description string so we can add a suffix.

    Juri> Meanwhile, I pushed the fix based on your recommendations on
    Juri> emacs-devel.  Feel free to improve it if your test reveals a
    Juri> problem.

It works fine for me. Of course, now that Iʼm using it, Iʼd like a
'make-frame-on-current-monitor' :-) I guess I can iterate over the
frames elements of 'display-monitor-attributes-list', unless thereʼs a
'what monitor is this frame on' primitive.

Robert