Display separators on tty menus with display table entries

Display separators on tty menus with display table entries

* src/xdisp.c (display_tty_menu_separator): Lookup separator char in
standard-display-table, make a string, and display that using
display_string.
* src/xdisp.c (display_tty_menu_separator_char): New function.
(display_tty_menu_separator): Use it.
* lisp/disp-table.el (display-table): Increase from 12 to 18.
(box-double-vertical, box-double-horizontal, box-double-down-right)
(box-double-down-left, box-double-up-right, box-double-up-left): New
symbols for extra slots.
(display-table-slot, set-display-table-slot): Change doc string.
(describe-display-table): Describe new slots.
(standard-display-unicode-special-glyphs): Define new slots.
* src/disptab.h (DISP_TABLE_P): Add enumerators.
(DISP_TABLE_EXTRA_SLOTS): Define based on enum box.
* src/dispnew.c (produce_box_glyphs): Add new enumerators to switch to
make it exhaustive.
* src/xdisp.c (display_tty_menu_separator): Use BOX_DOUBLE_HORIZONTAL
for '=' if present.
* doc/lispref/display.texi: Add documentation.
* etc/NEWS: Mention in NEWS.

(cherry picked from commit 4e78a3e117f4ca0b6b9f3b7a2d7919cb5b2e0295)