Use *load-truename* as a base for asdf:*central-registry* dirs

This branch uses *load-truename* of the toplevel .asd file to construct directories to be added to asdf:*central-registry*. This should make the system load correctly regardless of it's location in the filesystem.

Unfortunately, leaving the trailing slashes off the pathnames makes it cranky. This is probably fixable, but I didn't quite have time to address it.