Many improvements to the debugger, including cosmetic changes,

Many improvements to the debugger, including cosmetic changes,
separation of reduction zero from the subproblem, addition of
RETURN-TO command, variables to choose window configuration and when
to automatically destroy the debugger buffer, and more-subproblems
notification on the modeline instead of in the buffer.

In addition, various bug fixes and more of Brian's RMAIL enhancements.