the former is default now

- although not necessary, it should be there for completeness and for the sake
  of potential alternative implementations of greenlet

- do not use Pool as it's already demoed in dns_mass_resolve.py

- the default value is "gevent x.y Python x.y"

--HG--
extra : transplant_source : %19%0C%BAr%3D%88%0E%84%07%A7%B6T%EA%C2%28%A8%D4%FFe%9D

--HG--
extra : transplant_source : %E47%D6%993%F2%0D%5B%3A%83%05%80K%B9cH%A0%D4%D0L