Make defvar affect the default binding outside of any let.

Make defvar affect the default binding outside of any let.
* src/eval.c (default_toplevel_binding): New function.
(Fdefvar): Use it.
(unbind_to, backtrace_eval_unrewind): Do a bit of CSE simplification.
(Fdefault_toplevel_value, Fset_default_toplevel_value): New subrs.
(syms_of_eval): Export them.
* src/data.c (Fdefault_value): Micro cleanup.
* src/term.c (init_tty): Use "false".
* lisp/custom.el (custom-initialize-default, custom-initialize-set)
(custom-initialize-reset, custom-initialize-changed): Affect the
toplevel-default-value (bug#6275, bug#14586).
* lisp/emacs-lisp/advice.el (ad-compile-function): Undo previous workaround
for bug#6275.
* test/automated/core-elisp-tests.el: New file.