* lisp/custom.el: Avoid adding vars to load-history multiple times

* lisp/custom.el: Avoid adding vars to load-history multiple times

Avoid the abuse of (eval `(defvar ...)) which tends to end up
adding redundant entries in `load-history`, as discussed in
https://lists.gnu.org/r/help-gnu-emacs/2019-03/msg00237.html

(custom-initialize-default): Don't add to load-history.
(custom-declare-variable): Use internal--define-uninitialized-variable
and only add the var to load-history once.  Do it before calling
`initialize` so the special-variable-p flag is set.

* src/eval.c (Finternal__define_uninitialized_variable): New function.
(Fdefvar, Fdefconst): Use it.
(syms_of_eval): Defsubr' it.