Header guard for lin.h and checksum usage on diagnostic frames fixed.

[ChrisPHL]

[philwmcdonald]

Additionally in Lin::recv(): if (dataChecksum(message,nBytes,idByte) == cksum) bytesRcvd = 0xff; should probably be: if (dataChecksum(message,nBytes,(addr == 0x3d) ? 0: idByte) != readByte) bytesRcvd = 0xff;