Use a simple keyword for a non-nil argument

Use a simple keyword for a non-nil argument

The second argument of `run-with-idle-timer' is Boolean, i.e. only nil
and non-nil values are distinguished.  Passing a number here is
confusing.  Pass a descriptive symbol instead.

* lisp/frame.el (blink-cursor-mode, blink-cursor-check): Use
:repeat symbol instead of number for second argument of
`run-with-idle-timer'