ENH/BUG: support negative m in elliptic integrals

Prior, ellipkinc, and ellipeinc silently returned garbage for
negative m.  Both functions are still inaccurate due at extremely
large phi. 

I can upload some measurements on the accuracy for negative m tomorrow, but it seems to be fine.

Maybe this fixes gh-1431, if we're saying that's about negative m, or not if that's for complex support.
Commented-out code. No test coverage for this branch