Add hooks for extending arglist lookup.

Useful for example in [slime proxy](https://github.com/3b/slime-proxy), which wants to show arglists for parenscript functions/macros in addition to or instead of those defined in the host lisp.
I think my intent was that `%arglist` would always be last so it returning `:not-available` would be OK, though from looking at a few, the behavior of the backends didn't seem consistent. Some return `:not-available` when there is no function, some error in that case (sbcl), some just return NIL when a function exists but doesn't have any arglist data stored (sbcl again).