2.26.71: Fix obvious breakage on SBCL.

Make default-directory a root pathname, remove it from source-registry.
Have it use lisp-implementation-directory.
Remove any attempt to use the "current directory".
Users who want that can still try to
	(pushnew '*default-pathname-defaults* asdf:*central-registry*)

Rename tmp/ to build/ and move asdf.lisp there. Update asdf.asd.
Rename implementation.lisp to compatibility.lisp.
Move *asdf-verbose* and asdf-message to upgrade.lisp