Avoid redefining some C defcustoms.

Avoid redefining some C defcustoms.

* lisp/frame.el (show-trailing-whitespace, auto-hscroll-mode)
(display-hourglass, hourglass-delay, cursor-in-non-selected-windows):
Don't redefine things that are defined in C.
* lisp/cus-start.el: Also handle :risky, :safe, :set, and :tag.
(show-trailing-whitespace, auto-hscroll-mode)
(display-hourglass, hourglass-delay, cursor-in-non-selected-windows):
Set up the appropriate custom properties.