Implement Isearch in comint input history. (Bug#3746)

Implement Isearch in comint input history.  (Bug#3746)
* comint.el (comint-mode): Add `comint-history-isearch-setup' to
`isearch-mode-hook'.
(comint-history-isearch): New defcustom.
(comint-history-isearch-backward)
(comint-history-isearch-backward-regexp): New commands.
(comint-history-isearch-message-overlay): New buffer-local variable.
(comint-history-isearch-setup, comint-history-isearch-end)
(comint-goto-input, comint-history-isearch-search)
(comint-history-isearch-message, comint-history-isearch-wrap)
(comint-history-isearch-push-state)
(comint-history-isearch-pop-state): New functions.