bindings/python: Add priv_drop_user and priv_drop_group

[somechris]

This gives Python users control over which user/group the process is dropped into.

[hzeller]

Sorry for the delay, just going through old PRs :)
Could you re-base as the Python bindings have been touched recently.

[somechris]

Will do, but it'll take me a few days to get to it.

[hzeller]

The generated files are not included anymore to avoid staleness so only changes in the *.pyx need to be submitted. So I suspect it will be fairly straigthforward to update the patch.

[somechris]

Could you re-base as the Python bindings have been touched recently.

Done.

Thanks for removing the core.cpp. It's so much simpler without it! \o/

[hzeller]

Thanks, merged!