bug#7567: Please add a history variable to read-regexp

[Juri Linkov]

> So after reading the rest of the thread, I think I finally understand
> that you want to "add a history *argument* to read-regexp", right?
> Then it sounds perfectly acceptable.

For consistency, `read-regexp' could have the same function signature
as other minibuffer reading functions, e.g. like `read-string':

  (read-string PROMPT &optional INITIAL-INPUT HISTORY DEFAULT-VALUE 
INHERIT-INPUT-METHOD)

  (read-regexp PROMPT &optional INITIAL-INPUT HISTORY DEFAULT-VALUE 
INHERIT-INPUT-METHOD)