Small refactorings:

* Store the send & recv buffer sizes in #state so that nodeup events
that arrive after a set_dist_buf_sizes() call will use the specified
sizes rather than OTP app environment var defaults.

* Create get_riak_env_vars/0 helper function and export it for debugging use
After refactoring further, it's only used once internally, so if YMMV,
I'm fine with inlining this function's code and removing the export.

* Add explicit buffer size args to set_port_buffers().
Variable renaming to match inet:setopts() option names