SNR support for all TBS tuners

Drivers fot TBS tuner support SNR
this change work for all models ;)
dvb: re-instate some of the form scanning code.

Mux TSID and ONID can now be updated from other tables if, and only if,
they values are not yet set. Values from the NIT will ALWAYS override
existing values, as they're hopefully more reliable.
dvb: close dvr device on each re-tune
dvr: don't run postproc script unless there is actually a file to process
docs: updated the README and add markdown
Merge remote-tracking branch 'origin/pr/240'