New value 'permanent-only' for defcustom :local keyword

New value 'permanent-only' for defcustom :local keyword

* lisp/custom.el (custom-declare-variable): Make the :local keyword
accept the symbol 'permanent-local', meaning that the variable is
permanent but not marked as automatically buffer-local.
(defcustom):
* doc/lispref/customize.texi (Variable Definitions): Document the
above change.

* test/lisp/custom-tests.el
(custom-tests-defcustom-:local-keyword): New test.

(cherry picked from commit 9ad73f92617da1286724f9774b34bb7ce6a4299d)