channels file needs squelch setting

[ZeroChaos-]

Systems such as the frs example may have channels spread across multiple SDRs, especially for rtl-sdr and other fairly low rate SDRs. When this happens, gain may need to be different between the sources, which means squelch also needs to be different.
I currently solve this by having one system per gain but it would be much cleaner to define one system and allow adjusting of gain in the channels file.

[ZeroChaos-]

complete, in 5.0 release