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.
Either way is OK, though probably also should make it more clear what `:not-available` means in either case, if changing the backends.  
There are 3 cases that need distinguished:

* function is known and arglist is known to be empty
* function is known, but nothing is known about the arglist
* function isn't known

In first 2 cases it should stop, in 3rd it should check next hook function.
Not sure which of the last 2 `:not-available` was intended to indicate, or if the function is even intended to be called in the 3rd case.