* nsselect.m: pasteboard_changecount is new.

* nsselect.m: pasteboard_changecount is new.
(ns_store_pb_change_count, ns_get_pb_change_count)
(ns_get_our_change_count_for): New functions.
(ns_string_to_pasteboard_internal): Correct comment.
type => gtype in eassert, Call ns_store_pb_change_count.
(Fns_own_selection_internal): Remove data, use value.
(Fns_disown_selection_internal, Fns_selection_owner_p): Replace
Vselection_alist check, with change count check.
(Fns_get_selection): Initialize val to Qnil.  Only get local
selection if change counts match (Bug#18799).
(nxatoms_of_nsselect): Initialize pasteboard_changecount.