Folding hands complex exponentiations with constant arguments off to the
native libm, and on a least on host, this can produce spurious warnings
about division by zero and invalid arguments. Handle the case of a zero
base specially to avoid that, and also emit better warnings for the
undefined 0.**0 and (0.,0.)**0 cases. And add a test for these warnings
and the existing related ones.
a93d843ab3