Modified automatic cq zone nr copy to comment variable, if that's not empty

- If the user modified the exchange field, which was filled out by Tlf from
  cty.dat, then Tlf rewrite the original value if switching bqack forth
  between call input and comemtn field.
Added extra check for exchange field for CQ-WW-RTTY

- CQWW RTTY contest requires the two letter province for US and Canada
  stations in the exchange.