Improved sanitary facilities

[MOON at MIT-AI (David A. Moon)]

SYstem 1.0 with microcode 451, about to be installed on all machines,
contains a more or less working garbage collector.  It is not turned
on initially; if you feel like getting screwed, invoke the GC-ON function.
GC-OFF turns it off.  Note that once on it stays on even through re-booting
of the machine.

I welcome reasonably-specific reports of problems.