Until now gpython was always activating gevent on startup + adding
goodness such as "always UTF-8 default encoding"; go, chan, b/u and
friends available from builtin namespace, etc... While those goodness
are sometimes useful on their own, it is not always appropriate to force
a project to switch from threads to gevent.

For this reason add a flag to select which runtime - either gevent
or threads - gpython should use.

	gpython -Xgpython.runtime=gevent	selects gevent,

while

	gpython -Xgpython.runtime=threads	selects threads.

Gevent remains the default.

It is also possible to specify desired runtime via $GPYTHON_RUNTIME
environment variable.

/reviewed-on nexedi/pygolang!5