Improve yanking in the middle of comint process output

Improve yanking in the middle of comint process output

* lisp/comint.el
(comint--unmark-string-as-output): New function to remove unwanted
properties from text yanked from comint buffers.

(comint-mode): Use it as a 'filter-buffer-substring-function'.

(comint-output-filter): Set 'insert-in-front-hooks' text property on
process output such that text yanked by the user in the middle of
process output is marked as process output (Bug#3735).

(comint--mark-as-output): New function.
(comint--mark-yanked-as-output): New function.