Add new config value TRXStbyPWR

This is the power of the TRX if the KPA500 is in standby modus. Can be reed from the config file once the program is started. Important for the case if the program is closed while the KPA500 is in OPER stat. Those by the next start of the program the TRX PWR is the same in STBY and OPER.

This may fix issue https://github.com/DM5EA/hamlib2KPA500/issues/14
