Fix caller- vs callee-save comment in CHERI invoke code (spotted by
Robert Norton).
Now that we have more kernel stack/pcb space, extend the CHERI trusted stack
for each thread from two entries (enough for a sandbox to invoke the system)
to 4 (enough to do something more).  A better long-term design is required
here, but this will work for now and allow us to express more complex/
interesting models.