Add :timeout option to condition-wait.

Implemented for: ABCL, Allegro, CLISP, Clozure, ECL,
LispWorks 6.0 and later, SBCL, SCL.

Other implementations signal an error when condition-wait
is called with a non-nil :timeout argument. A style-warning
may be issued during compilation.

Thanks to Thomas Monro for surveying this functionality
across Lisp implementations.