Support setting PAGER=cat in comint.el (bug#62958)

Support setting PAGER=cat in comint.el (bug#62958)

Paging can be undesirable in comint-derived commands such as
async-shell-command and M-x shell.  It is a frequent footgun for new
Emacs users when they try to run commands which start a pager in such
modes.

Simply adding (setenv "PAGER" "cat") globally is not correct, since
that will break modes like term, which support paging quite well.
It's only and exactly the comint-derived modes which don't need
paging.

* lisp/comint.el (comint-pager): Add. (bug#62958)
(comint-exec-1): Use comint-pager to set PAGER.