gpython: Add way to run it with threads runtime
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.
The main change is in patch4. Patch3 is purely preparatory. Patch2 adds support for -V/--version along the way. Patch1 fixes -c/-m handling.