Fixed problems with new "toothbrush" stuff, especially when using SBCL.

Thanks to Jared for pointing out problems with the new "toothbrush"
stuff.  Now, regressions will skip books/system/toothbrush/ unless
environment or 'make' variable LISP is defined, in which case LISP
should designate a Lisp executable.  Also, changes have been made that
avoid warnings as well as a locked-package error when using SBCL.

Note that regressions don't touch books/system/toothbrush/ unless the
environment (or 'make') variable LISP is set, in which case it should
designate a Lisp executable.

If LISP is set, then it no longer needs to designate CCL or SBCL,
though these are still recommended since the toothbrush mechanism
still loads ACL2 source files, not compiled files.  (Also, not all
Lisps have been tested; in particular, CLISP caused a failure, but I
plan to look into that.)