Stop subprocesses from using inherited or default PAGER

Stop subprocesses from using inherited or default PAGER

At startup, set PAGER to "cat" so that any inherited or
default value of PAGER does not affect subprocesses of
Emacs.  Pagers generally won't work when a subprocess
runs under Emacs.
Users can use 'comint-pager' (or other customizations)
to tell subprocesses to use a different specific pager.
* lisp/startup.el (normal-top-level): Set PAGER to 'cat',
if 'cat' is available.  (Bug#72426)

(cherry picked from commit 7811a7d38bb7cb303dc66efa02eb95e75a03f39d)