Support additional rigctld commands

[srcejon]

The attached patch adds support for additional rigctrld commands: set_mode, get_mode, set_powerstat and get_powerstat. This should eventually allow gpredict (and other s/w) to set the correct mode (E.g. FM/AM) and bandwidth, for the satellite that is being tracked and also disable the radio when no satellites are visible.

It also changes the get_freq command to return the actual current frequency of the radio, rather than the last frequency that was set, in case it has been manually changed.

additional_rigctld_commands.txt