ENH: add the limit shape c=0 to genpareto distribution

For c=0, genpareto is equivalent to the exponential
distribution.
BUG: use special.log1p instead of np.log1p

np.log1p(np.inf) is reported to produce nans on some platforms,
see numpy issue gh-4225
BUG: use scipy.special.expm1 instead of np.expm1
ENH: vectorize genpareto._munp

implementation is by @pbrod