Persist user-configured *quicklisp-directory* location

[Hexstream]

Right now the CL part of the CLHS wrapper will always fall back to the default quicklisp directory location when loaded for the first time in a CL image, so those with a "non-standard" location will need to (setf clhs:*quicklisp-directory* "/path/to/quicklisp") each time they use that part (which is presumably not often, only for installation/reinstallation/upgrade).

Maybe I should provide a way to persist this user preference...

[Hexstream]

In version 0.6.1, the clhs wrapper will attempt to retrieve the "authoritative" quicklisp directory location from ql:quicklisp-home, if possible, to use as default quicklisp directory.

This should normally always be successful, so there's not much point in persisting this, and in fact I probably wasted my time by exposing functionality to set the quicklisp directory and referencing it in the setup help. Oh well!