Check asdf version, gracefully degradate to swank-loader.lisp, fix bugs

ASDF is the default and is passing the following test scenarios:

LISP_BIN=cmucl make clean check
LISP_BIN=cmucl make clean check-fancy
LISP_BIN=alisp make clean check ;; alisp is Allegro Common lisp

SBCL is notably not passing *any* tests when loaded with ASDF.due to
a strange "11 nested errors" problem which must be investigated.

* slime.el (slime-init-using-asdf): Fall back gracefully to
swank-loader.lisp.

* swank.asd (:swank): Simplify with tips from Fare.

* swank.lisp (*require-module*): INTERN takes strings, not
symbols.

Conflicts:
	slime.el