Limit the amount of text we examine when looking for password prompts

Limit the amount of text we examine when looking for password prompts

Both Comint and Eshell do this, and it can significantly slow down
commands that write a lot of output (bug#71576).

* lisp/comint.el (comint-password-prompt-max-length): New variable...
(comint-watch-for-password-prompt): ... use it.  Additionally, use the
matched result for the Emacs-based password prompt.

* lisp/eshell/esh-mode.el (eshell-password-prompt-max-length): New
variable...
(eshell-watch-for-password-prompt): ... use it.

* etc/NEWS: Announce this change.