Use separate packet out for learning switch examples

Currently learning switch C example sends Flow Mod with valid buffer_id.
But the style is noted as dangerous in [1].
This patch separates Flow Mod and Packet Out.

And Ruby example sends Packet-Out after Flow Mod without data.
This patch solves the issue too.

[1] https://github.com/yasuhito/trema-book/blob/master/ja/openflow_spec.re