New file lisp/emacs-lisp/debug-early.el for backtraces in early bootstrap

New file lisp/emacs-lisp/debug-early.el for backtraces in early bootstrap

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.