add support for Sierra Wireless qcserial NMEA-0183 interface

[dangowrt]

Sierra Wireless EM 74xx modems come with a serial port outputting NMEA-0183 GPS sentences. In order to make it work, the magic string '$GPS_START' needs to be written to the modem, as only then the modem firmware starts sending NMEA-0183 output. Add option 'sierragpsstart' which if set to anything else than 0 will make kplex send the magic string when the device is opened.

Fixed #53