Add hooks for extending arglist lookup.

* contrib/swank-arglists.lisp (*valid-operator-symbol-p-hooks*):
New variable.
(valid-operator-symbol-p): use it

* swank-backend.lisp (%arglist): rename interface from arglist
(*arglist-hooks*): new variable, default to %arglist
(arglist): new function, call functions in *arglist-hooks*

* swank-abcl.lisp (%arglist): renamed from arglist
* swank-allegro.lisp (%arglist): renamed from arglist
* swank-ccl.lisp (%arglist): renamed from arglist
* swank-clisp.lisp (%arglist): renamed from arglist
* swank-cmucl.lisp (%arglist): renamed from arglist
* swank-corman.lisp (%arglist): renamed from arglist
* swank-ecl.lisp (%arglist): renamed from arglist
* swank-lispworks.lisp (%arglist): renamed from arglist
* swank-sbcl.lisp (%arglist): renamed from arglist
* swank-scl.lisp (%arglist): renamed from arglist