Use CLIPBOARD and PRIMARY on NS (Bug#6677).

Use CLIPBOARD and PRIMARY on NS (Bug#6677).

* lisp/term/ns-win.el (ns-get-pasteboard, ns-set-pasteboard): Use
QCLIPBOARD instead of QPRIMARY (Bug#6677).

* src/nsselect.m (QCLIPBOARD, NXPrimaryPboard): Define.
(symbol_to_nsstring): Map QCLIPBOARD => NSGeneralPboard,
QPRIMARY => NXPrimaryPboard.
(ns_string_to_symbol): NSGeneralPboard => QCLIPBOARD,
NXPrimaryPboard => QPRIMARY.
(nxatoms_of_nsselect): NXPrimaryPboard = PrimarySelection,
NXSecondaryPboard = SecondarySelection.
(syms_of_nsselect): Intern QCLIPBOARD (Bug#6677).