Finished conversion from methods to functions with defaults (Stelian Ionescu <sionescu@common-lisp.net>)

Centralized loading and checking of impl-specific thread libraries (Stelian Ionescu <sionescu@common-lisp.net>)

Added a file containing the MIT license and a copyright and license notice in
each source file

Exported *supports-threads-p* so that apps can check that variable.
Some other small changes to make it load and 'work' on clisp.