BUG: stats: correct value of powerlaw.logpdf(0, 1) is 0. Closes gh-3986.
BUG: stats: Better handling of x=0 in the 'exponpow' distribution.

exponpow.logpdf(0, 1) now returns 0.0 instead of nan.

When b < 1, exponpow.pdf(0, b) and exponpow.logpdf(0, b) now
return inf instead of nan.

Closes gh-3982.
BUG: stats: gengamma.pdf(0,1,1) now returns 1.0 instead of nan.

Closes gh-3985.
Merge pull request #4000 from WarrenWeckesser/stats-powerlaw-nan

BUG: stats: Fix edge-case handling in a few distributions.