[BUG] RTL_PPM is hardcoded, but differs per RTL-SDR stick

[AgriVision]

The rtl ppm offset, which differs for each individual stick is currently a hardcoded value in sat.sh.
I would propose to place it in user.conf and add some documentation there or in the README on how to obtain this value, e.g. rtl_test, or https://www.rtl-sdr.com/tag/ppm/, or other sources.

[stdevPavelmc]

Good catch you are right about it, and fixing is simple, WIP.

[stdevPavelmc]

Done, upgrade as usual...