BUG: special: fix asymptotic expansion in rgamma
BUG: special: fix several issues in poch

Handle better the cases where the gamma functions in the definition hit
poles or over/underflow.

Fixes gh-1437
GEN: special: regenerate files
Merge pull request #2735 from pv/special-rg

BUG: special: Fix asymptotic behavior of rgamma and reimplement poch