bug#70587: More buffers that ought to be read-only

[Dan Jacobson]

diff-default-read-only is a variable defined in ‘diff-mode.el’.
Its value is nil
If non-nil, ‘diff-mode’ buffers default to being read-only.

That's funny, it seems they are already read-only, without setting this
variable. Maybe this variable can be retired.

The *compilation* buffer of M-x compile is also read-only. Good.

But how about *Shell Command Output*, and errors buffers, they should be
read-only by default too.

Why? Because tons of times we realize for the last half hour we have
been editing our program's output buffer instead of the source code.

(Often due to using sh -x or make (without -s) which echo command
lines.)

emacs-version "29.3"