disables adaptive gain.
This is needed for the case where a v5 package with an unmodified
config file is upgraded. dpkg will install the v6 default config
file automatically so it will already be a v6 config by the time
that upgrade-config runs, but we do still need to go in and
disable adaptive gain.
The legacy -10 value is still supported for compatbility with old
configs, but new configs should just select an appropriate gain e.g.
60dB.
rtlsdr default gain continues to be the highest regular gain step (~ 49.6dB)
on upgrade to 6.0. (Not extensively tested yet)
We do this for two reasons:
1) the config file layout is completely different, it's error-prone
and tedious to force the user to manually transfer their customizations
on upgrade;
2) if there are user modifications to the config file, an upgrade triggered
remotely via piaware will prefer to keep the original config file intact
rather than installing the new version, and the old version just won't
work correctly with the new infrastructure.
Oliver Jowett