utils: convenience functions

Signed-off-by: Steve Markgraf <steve@steve-m.de>
rtl_adsb: fix threading

Signed-off-by: Steve Markgraf <steve@steve-m.de>
rtl_test: useful ppm

Signed-off-by: Steve Markgraf <steve@steve-m.de>
rtl_power: fix dBm math

Signed-off-by: Steve Markgraf <steve@steve-m.de>
rtl_fm: overhaul

Signed-off-by: Steve Markgraf <steve@steve-m.de>