4. Call INTERPRET which interprets boot source code until
   ASCII EOT (4) is met. This usually init drivers.
5. Initialize rdln buffer, _sys entry (for EMPTY), prints
   "CollapseOS" and then calls (main).
6. (main) interprets from rdln input (usually from KEY) until
   EOT is met, then calls BYE.

In RAM-only environment, we will typically have a
"CURRENT @ HERE !" line during init to have HERE begin at the
end of the binary instead of RAMEND.
