BUG: use correct MAXGAM in beta.c

AFAICT the correct constant is used everywhere in cephes.

Also try harder to use the best order of operations when
calculating beta from gammas.
BUG: don't try to calculate beta in betainc, just call it.

fixes gh-3761
Merge pull request #3779 from ewmoore/incbet_fix

BUG: special: fix overflow problems in betainc, and improve precision of beta

Fixes gh-3761