lirc_imon: implement pad2keys for older receivers
lirc_imon: implement pad2keys for older receivers

Makes behavior of both older (id 0xffdc vfd) and newer imon
receivers the same with respect to converting the pad into
keypresses in a sane way for use with vdr, mythtv, etc.

From http://www.brakemeier.de/electronics/vdr/lirc-imon.html,
pulled in at Michael's request.

stb-tester/lirc,stb-tester/lirc,stb-tester/lirc,stb-tester/lirc,stb-tester/lirc,stb-tester/lirc