Fix the deugger (fixing Trac #8557)

The runtime debugger (which has not received any love from anyone
for many years) looks wrong to me; it was failing to instantiate the
outer foralls of a variable when called from :force, which calls
cvObtainTermFromId, which calls cvObtainTerm

I simplified the code too.  But I'm flaky on how this debugger stuff
is really supposed to work, so I'm partly guessing.  Tests pass though.