Fix handling of defcustom :local tag

Fix handling of defcustom :local tag

For discussion, see the following emacs-devel thread:
https://lists.gnu.org/r/emacs-devel/2020-11/msg00734.html

* lisp/custom.el (custom-declare-variable): Delay call to
make-variable-buffer-local until after user option has been
initialized with a value.  Otherwise the user option may be
initialized to nil.
* test/lisp/custom-tests.el (custom--test-local-option)
(custom--test-permanent-option): New :local user options.
(custom-test-local-option): New test for defcustom :local keyword.