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.
Well, let's fix the inconsistency. Do you have any objection against the "arglist or `:not-available`" protocol? If not, I'll go through the backends and make them respect that protocol. (I'm OK with the gethash-like protocol too, if you prefer that...)