Make DUMP1090 gain easily accessible

[SharkyMoto]

I'm running into stability issues with my nooelec v5 stick since it gets too hot with max gain using ads-b feeder - this option should be easier accessible and documented, since this happens to some people. changing the gain should fix the overheating issue.

[myromeo]

Same dongle same issue, did you find a work around? Ideally we should be able fuse auto gain, dump1090 I think calls this with the arg —gain -10 however this causes dump1090 to fail to start, unknown command