Also check SWANK version and interfaces with ASDF loader

* swank.lisp (before-init): don't take VERSION arg.
(slime-version-string): move it here.
(init): Call BEFORE-INIT.
(before-init): don't redo if :SWANK in *FEATURES*
(start-server): call INIT unless :SWANK in *FEATURES*

* swank-loader.lisp (slime-version-string): remove it.