src/dvb/diseqc: add method for EN50494 SCR tuning

EN50494 ("Unicable") basically sends special DiSEqC commands to the
LNB / switch matrix to select a channel and frequency, later the
frontend is tuned to a fixed freq.
This blends in pretty easily with diseqc_setup, so just add a similar
function named en50494_setup.
src/dvb: implement EN50494 SCR tuning
webui: add EN50494 config bits
dvb: shut down en50494 SCR on power down event