[Cluster-tech] Add CRC to PC9x

[Lee Sawkins]

Hi Dirk

Is it going to cause you any problems if I add a CRC immediately after
the last "^" character in the PC9x sentences?  This would be a PKZip
compatible CRC32.  When I split up the current PC9x sentences I do not
process data after the last "^" as it is always empty.  Does Spider do
the same?  This CRC would not include the hops count, since it changes.

The reason for adding this is to find and discard corrupted packets. 
This would help stop looping and prevent erroneous data from entering
the user/node tables.

It would be great if you could do the same with your PC9x packets.

Lee