Return the "vendor ID" string of the X server of display TERMINAL.
(Labeling every distributor as a "vendor" embodies the false assumption
that operating systems cannot be developed and distributed noncommercially.)
The optional argument TERMINAL specifies which display to ask about.
TERMINAL should be a terminal object, a frame or a display name (a string).
If omitted or nil, that stands for the selected frame's display.
(fn &optional TERMINAL)