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.
BUG: ellipkinc(p, 1) imprecise.

The old expression was imprecise for small phi and for phi very
close to pi/2.
TST: special: enable some mpmath test for incomplete elliptic integrals E and K
TST: enable ellipkinc tests against boost data
TST: Add tables driven tests of ellip(k,e)inc values with neg m
ENH: fix failing argument reduction for large phi. (ellip(e,k)inc)

The floating point multiples of pi/2 won't fit into an int.
Argument reduce into a double instead.
ENH: Improve precision of ellipeinc for small p, m in [0,1].
DOC: Improve docstrings for elliptic integrals.
BUG: full C code coverage of elli(e,k)inc tests.

Also address review comments.
Merge pull request #3767 from ewmoore/ellip_neg

BUG: special: support negative m in elliptic integrals