Move batch backtrace code to `top_level_2`

Move batch backtrace code to `top_level_2`

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.