Better multi-line input support in comint.el

Better multi-line input support in comint.el

Revert the change of comint-line-beginning-position callers,
and modify comint-line-beginning-position instead.

* lisp/comint.el (comint-history-isearch-search)
(comint-history-isearch-message, comint-history-isearch-wrap):
Use comint-line-beginning-position instead of field-beginning.
(comint-send-input): Use either end-of-line or field-end
depending on comint-use-prompt-regexp.
(comint-line-beginning-position): Search backward
for comint-prompt-regexp if comint-use-prompt-regexp is non-nil.
Use field-beginning instead of line-beginning-position
if comint-use-prompt-regexp is nil.

Fixes: debbugs:19710