Whether the current Emacs process owns the given X Selection.
The arg should be the name of the selection in question, typically one of
the symbols `PRIMARY', `SECONDARY', or `CLIPBOARD'.
(Those are literal upper-case symbol names, since that's what X expects.)
For convenience, the symbol nil is the same as `PRIMARY',
and t is the same as `SECONDARY'.
TERMINAL should be a terminal object or a frame specifying the X
server to query. If omitted or nil, that stands for the selected
frame's display, or the first available X display.
On Nextstep, TERMINAL is unused.
(fn &optional SELECTION TERMINAL)