Initialise tlf's node designator (THISNODE=...) to 'A'

- Even if you do not use the LAN features and forgot to name your node
  in logcfg.dat the node designaotr is used internally for adding a
  QSO to the bandmap.
- Further add some check logic to allow only designators 'A'..'H'
document default setting for node designator in man page