[aprssig] Voice Repeater Frequency SPec

[Robert Bruninga]

OK, the APRS Frequency spec has been updated.

In addition to the existing Tnnn, Dnnn, these were added:

1750 and NoPL.

http://www.ew.usna.edu/~bruninga/localinfo.html
http://www.ew.usna.edu/~bruninga/aprs/freqspec.txt

And if you have no PL and no tone, then those 4 bytes may be
used for other purposes.

Bob, WB4APR