Reimplement `backtrace-on-redisplay-error` using `push_handler_bind`.
This moves the code from `signal_or_quit` to `xdisp.c` and
`debug-early.el`.
* lisp/emacs-lisp/debug-early.el (debug-early-backtrace):
Add `base` arg to strip "internal" frames.
(debug--early): New function, extracted from `debug-early`.
(debug-early, debug-early--handler): Use it.
(debug-early--muted): New function, extracted (translated) from
`signal_or_quit`; trim the buffer to a max of 10 backtraces.
* src/xdisp.c (funcall_with_backtraces): New function.
(dsafe_calln): Use it.
(syms_of_xdisp): Defsym `Qdebug_early__muted`.
* src/eval.c (redisplay_deep_handler): Delete var.
(init_eval, internal_condition_case_n): Don't set it any more.
(backtrace_yet): Delete var.
(signal_or_quit): Remove special case for `backtrace_on_redisplay_error`.
* src/keyboard.c (command_loop_1): Don't set `backtrace_yet` any more.
* src/lisp.h (backtrace_yet): Don't declare.
Move ad-hoc code meant to ease debugging of bootstrap (and batch mode)
to `top_level_2` so it doesn't pollute `signal_or_quit`.
* src/lisp.h (pop_handler, push_handler_bind): Declare.
* src/keyboard.c (top_level_2): Setup an error handler to call
`debug-early` when noninteractive.
* src/eval.c (pop_handler): Not static any more.
(signal_or_quit): Remove special case for noninteractive use.
(push_handler_bind): New function, extracted from `Fhandler_bind_1`.
(Fhandler_bind_1): Use it.
(syms_of_eval): Declare `Qdebug_early__handler`.
* lisp/emacs-lisp/debug-early.el (debug-early-backtrace): Weed out
frames below `debug-early`.
(debug-early--handler): New function.
The `noerror` arg of `require` doesn't do what Stefan naively thought
when he wrote this code, so use `condition-case` instead.
Also check for `cl-defmethod` since `cl-prin1` can't be loaded before
that anyway.
* lisp/emacs-lisp/debug-early.el (debug-early-backtrace):
Escape newlines to and bytecodes to make backtraces slightly more
readable. Use `cl-prin1` when available.
This is also used in batch mode in general.
* lisp/debug-early.el (debug-early-backtrace, debug-early): New functions.
* lisp/loadup.el (top level): Load debug-early.el as first file.
* src/eval.c (signal_or_quit): Remove the condition in the batch mode section
of not being in dumping or bootstrap, since it is no longer needed. Test that
'debug-early's symbol-function is bound. Ensure there is enough working space
in specpdl and eval_depth.
(syms_of_eval): New DEFSYM for Qdebug_early. Initialise Vdebugger to
Qdebug_early rather than Qnil.
