Whether there is an owner for the given X selection.
SELECTION should be the name of the selection in question, typically
one of the symbols `PRIMARY', `SECONDARY', `CLIPBOARD', or
`CLIPBOARD_MANAGER' (X expects these literal upper-case names.) 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)