New switch allows dump1090 to work with/without dongle

* With --net-only switch dump1090 wouldn't look for an RTLSDR device

* Without --net-only switch dump1090 would abort if it couldn't find an RTLSDR
device.

* Now with --net --no-rtlsdr-ok dump1090 will try to find an RTLSDR device but
go ahead and start either way.