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