Commit Briefs
clean up when finished.
don't set PLAN9 don't set PLAN9
Checkpoint.
Add disk caching code and first draft of fractional index.
Changes for Mac OS X. Most important is stack sizes in samterm,
which were completely bogus. (Libthread used to ignore them but not anymore. Maybe we really should ignore them, but that breaks Venti, which needs *really* big stacks.)