bug#26947: Preserve read-only when debugger with let-bound inhibit-read-only t.

[Keith David Bershatsky]

Entering the debugger when inhibit-read-only is let-bound to `t` causes all 
buffers to be read/write even though they may have `buffer-read-only` set to 
`t`.

Step #1:  Launch with emacs -q.

Step #2:  Navigate to the `*scratch*` buffer and set to read-only with C-x C-q.

Step #3:  Evaluate:  (let ((inhibit-read-only t)) (error "stop"))

Step #4:  Select the window display the `*scratch*` buffer and type something.  
All open buffers are read/write.

Expected Behavior:  Protect buffers that have previously been set to 
`buffer-read-only` with `t`.

Workaround:  Add the following to the outset of the function `debug`:

(when inhibit-read-only
  (setq inhibit-read-only nil))

The workaround can be perfected to test for whether inhibit-read-only is 
let-bound to `t` and if it is then ...  I say that just in case someone wants 
`inhibit-read-only` to always be `t` and in that case the debugger shouldn't 
turn it off.

Here is the new thread on emacs.stackexchange.com:

https://emacs.stackexchange.com/questions/32829/inhibit-read-only-affects-all-buffers-when-backtrace-buffer-is-open

Thanks,

Keith