Add support for 6-symbol QTH locators

[sp9dev]

Hi! Could you add support for transmitting 6-symbol locators? Thanks!

[MichaelRenner]

An good idea. Support for type 2 and type 3 messages. Would include "long" call sign, e.g. if you are abroad.

[ComputerSmiths]

Yeah, I just brought my TAPR Hat to the Turks and Caicos, and discovered VP5/N1JBJ doesn't work. Also, my NTP server's gpsd gives me an 8-character grid square, so if I'm going to be doing two transmissions, I might as well narrow down exactly where the only WSPR transmitter in the TCI is 8*)