NOTE: this does not indicate full P25 data support as working yet; this commit refactors a lot of the baseline PDU handling code, and better handles both confirmed and unconfirmed PDU data blocks, there is still some more work to do, I just didn't want p25_data to start getting overly out of sync with master.
* rework modem protocol to support variable length frames up to 64k; refactor P25 PDU handling to support TIA-102 minimum 512 byte frames;
* fix PDU length its 512 bytes not 500; ignore instead of report invalid modem frames; add double length checking to HostSetup;
* add support to send P25 long frames to the modem;
* adjust FIFO size to 522 (fits a single PDU frame); remove extra debug displays; match fixed FIFO sizes between modem and host;
* correct weird inverted CRC-9 that sometimes happen;
* ensure netPDU is reset at the beginning of a network data call; fix lengh used by data block dumping;
* fix some buffer lengths;
Bryan Biedenkapp