Fix Issue 833 (#835)
* Correct 1F1[-n, -n, x]
To match what Mathematica does.
Fixes: https://github.com/boostorg/math/issues/829
* Pass policy in forwarding call to gamma_q.
* Fix bug in 1F1 recurrence relations when one term goes to zero.
Fixes issue with 1F1[-n, n, n].
Also reduce cancellation error in a = -1 special case.
Fixes https://github.com/boostorg/math/issues/833.
