scpi_tcp: split into scpi_tcp_raw and scpi_tcp_rigol

The current implementation is renamed to tcp-rigol as it seems to be
a Rigol proprietary protocol used only on Rigol VS5000 series.

A new tcp-raw implementation is introduced which simply carries raw SCPI
commands over TCP. It is probably a much more common protocol and it is
at least available on Rigol DS2000 series on port 5555.