Wrong value of background-mode on dark XTerm

[Juliusz Chroboczek]

Running 23.0.94.1 on an ``xterm -rv'' (reverse-video), I get

    (frame-parameter nil 'background-mode)
    ==> light

which oviously reads to almost illegible text in certain modes.

With recent XTerm versions, the background colour can be retrieved with
the escape sequence ``Operating System Controls''

  ESC ] 1 1 ; ? ESC \ 

which returns an escape sequence in the same format, but with the ``?''
replaced with an RGB specification

  ESC ] 1 1 ; rgb:0000/0000/0000 ESC \

I have no idea what this escape sequence does to older XTerms, but the
XTerm version number can be retrieved with ``Send Device Attributes''

  ESC [ > c

which returns something like

  ESC [ > 0 ; 2 4 2 ; 0 c

where the 242 is the XTerm patchlevel.

                                        Juliusz