Get rid of backend-dependent selection-handling functions for kill/yank
and make it generic instead by relying on the lower-level selection
management functions.

* select.el (select-enable-clipboard): Rename from
gui-select-enable-clipboard.
(select-enable-primary): Move from x-win.el and rename from
x-select-enable-primary.
(gui-last-selected-text): Remove.
(gui--last-selected-text-clipboard, gui--last-selected-text-primary):
New vars.
(gui-select-text): Rewrite, based on x-win.el's old x-select-text.
(gui-select-text-alist, gui-selection-value-alist): Remove.
(x-select-request-type): Move from x-win.el.
(gui--selection-value-internal): New function, taken from x-win's
x-selection-value-internal.
(gui-selection-value): Rewrite, based on x-win.el's old x-selection-value.
(gui-set-selection-alist): Rename from gui-own-selection-alist and
extend it to handle a nil value as a "disown" request.
(gui-disown-selection-alist): Remove.
(xselect-convert-to-delete): Adjust accordingly.
(gui-set-selection): Simplify accordingly as well.  Use dotimes.

* lisp/frame.el (gui-method): Use window-system rather than framep.
(gui-method-declare): The tty case is now nil rather than t.
(make-frame): Adjust accordingly.

* lisp/term/x-win.el (x-last-selected-text-primary)
(x-select-enable-primary): Remove (moved to select.el).
(x-select-request-type): Move to select.el.
(x-selection-value-internal, x--selection-value): Remove functions.
(gui-selection-value, gui-select-text): Remove moethods.
(gui-set-selection): Merge own and disown methods.

* lisp/startup.el (command-line): Adjust now that `gui-method' expects nil
for ttys.

* lisp/term/ns-win.el (ns-get-pasteboard, ns-set-pasteboard)
(ns-selection-value): Remove functions.
(gui-select-text, gui-selection-value): Don't define method any more.
(gui-set-selection): Merge the old own and disown methods.
(gui-selection-exists-p, gui-get-selection): Adjust to new name of
underlying C primitive.

* lisp/term/pc-win.el (w16-get-selection-value): Add dummy argument and drop
test of gui-select-enable-clipboard, to make it usable as
a gui-get-selection method.
(gui-selection-exists-p): Adjust to new name of C primitive.
(gui-set-selection): Merge own and disown methods.
(gui-select-text, gui-selection-value): Delete methods.
(w16--select-text): Delete function.

* lisp/term/w32-win.el (w32--select-text, w32--get-selection-value):
Delete function (move functionality into w32--set-selection and
w32--get-selection).
(gui-select-text, gui-selection-value): Don't define methods.
(w32--set-selection, w32--get-selection, w32--selection-owner-p):
New functions.
(gui-get-selection, gui-selection-owner-p, gui-selection-exists-p):
Use them.
(gui-selection-exists-p): Adjust to new name of C primitive.

* src/nsselect.m (ns_get_local_selection): Signal error rather than `quit'.
(Fns_own_selection_internal): Tighten scoping.
(Fns_selection_exists_p): Rename from Fx_selection_exists_p.
(Fns_get_selection): Rename from Fx_get_selection_internal.
(Fns_get_selection_internal, Fns_store_selection_internal):
Remove functions.
(syms_of_nsselect): Adjust accordingly.

* src/w16select.c (Fw16_selection_exists_p): Rename from
Fx_selection_exists_p.
(syms_of_win16select): Adjust accordingly.

* src/w32select.c (Fw32_selection_exists_p): Rename from
Fx_selection_exists_p.
(syms_of_w32select): Adjust accordingly.