Add customization data to several built-in variables.

Add customization data to several built-in variables.

* lisp/cus-start.el (gc-cons-percentage, exec-suffixes)
(dos-display-scancodes, dos-hyper-key, dos-super-key)
(dos-keypad-mode, debug-on-signal, vertical-centering-font-regexp)
(make-cursor-line-fully-visible, void-text-area-pointer)
(font-list-limit): Add customization data.

* doc/lispref/display.texi (Font Lookup):
* doc/lispref/frames.texi (Pointer Shape):
* doc/lispref/processes.texi (Subprocess Creation): Use defopt for options.