[aprssig] UIdigi and New n-N

[Robert Bruninga]

The Overlay for old PacComm Trace-only digi's that used
to have a "T" overlay should now be changed to a "P".

ALl of the settings for the PacComm callsign-substituting
TNC's to be somewhat compatible with the New n-N Paradigm
have now been added to the New-n-N Paradigm page.
After these changes are made, the overlay becomes a "P"
to let the users know...

BOb