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