Logging activity of scanned frequencies

When scan mode is in use, RTLSDR-Airband can optionally write a log message whenever there is an activity on any of the scanned frequencies:

Jan 17 11:25:32 pi rtl_airband[409]: Activity on 123.400 MHz

This feature is disabled by default. It can be enabled globally by setting log_scan_activity configuration parameter to true:

# global parameters go here
log_scan_activity = true;

# here goes devices configuration
devices: ({
( ... )
});