- it's no longer used by gevent itself

- add environ argument which can be used to override WSGI's environ passed to the handlers
- set log to sys.stderr if None is passed
- set wsgi.errors to stderr if it was not set otherwise; do not reference sys.stderr from base_env class variable

wsgi: pass server instance to handler's constructor to match pywsgi's API; log message to server's log rather than to stdout

GreenletSet: add explicit block=False to kill() and killone() calls where we need async calls, since the default now is to block