(comint-use-prompt-regexp-instead-of-fields): New variable.

(comint-use-prompt-regexp-instead-of-fields): New variable.
(comint-prompt-regexp, comint-get-old-input): Document dependence on
  comint-use-prompt-regexp-instead-of-fields.
(comint-send-input): Add `input' field property to stuff we send to
  the process, if comint-use-prompt-regexp-instead-of-fields is nil.
(comint-output-filter): Add `output' field property to process
  output, if comint-use-prompt-regexp-instead-of-fields is nil.
(comint-replace-by-expanded-history)
(comint-get-old-input-default, comint-show-output)
(comint-backward-matching-input, comint-forward-matching-input)
(comint-next-prompt, comint-previous-prompt):  Use field
  properties if comint-use-prompt-regexp-instead-of-fields is nil.
(comint-line-beginning-position): New function.
(comint-bol): Use comint-line-beginning-position.  Make ARG optional.
(comint-replace-by-expanded-history-before-point): Use
  comint-line-beginning-position and line-end-position.
(comint-last-output-overlay): New variable.
(comint-mode): Make `comint-last-output-overlay' buffer-local.