Add digiline support to mese tube

Channel is hard-wired as "tube" at the moment, because it's not obvious
how (or even if) to re-work the formspec to allow the channel to be
specified - it's already filling my screen.

To use, send a message on channel "tube", where the message is a table
of the following form: {"on", "on", "on", "on", "on", "on"} - always 6
entries, one corresponding to each row/tube connection, in the same
order as on the form. Each entry must be either "on" or "off". Invalid
messages are just ignored.