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.