Avoid some compiler warnings.

* swank-backend.lisp (utf8-encode): Avoid unreachable code
warnings.
(%utf8-encode): New helper.
* swank-sbcl.lisp (sbcl-wrap): Add ignore declaration.
Avoid some compiler warnings.

* swank.lisp (before-init): Don't call
warn-unimplemented-interfaces as it confuses first time users.
(thread-for-evaluation): Suppress unused var warnings.

* contrib/swank-asdf.lisp: Avoid unreachable code for version check.
(unique-string-list): Avoid unreachable code warnings.

* contrib/swank-fuzzy.lisp (*all-chunks*): Defvar it and declaim the type
to avoid type uncertainty warnings.

* contrib/swank-package-fu.lisp (export-structure): Suppress warning about
unknown function list-structure-symbols with #+.

* contrib/swank-presentations.lisp (*send-repl-results-function*): Defvar
it to avoid unknown var warning.
* Makefile (clean-fasls): Recognize more file extensions.