Fixed backspace and cmd_arg()

Fixed backspace handling  and cmd_arg() false positives
Added new NMEA types

Added new NMEA types: NMEA_SEN_PGACK and NMEA_SEN_PGTOP for antenna
sentences
Added antenna status command

Added antenna status command: PCMD_ANTENNA with OFF/ON/Q arguments
setNavThreshold() call fixed
Fixed command history and updated pmtk command

Fixed command history and updated pmtk command
Updated pmtk command description