API / opcode behavior change - find_global/get_global

[Leopold Toetsch]

* iff global lookup exceptions are off, a Null PMC is returned instead 
of
  Undef on lookup failure

This is necessary as a global .Undef with that global name could still 
exist, but the difference WRT non-existing global isn't even testable 
with 'defined'. Now ...

  g = global 'foo'
  if null g goto g_doesn_exist

... works for defined but .Undef vars too.

leo