Pad T_PositionMsg to a multiple of 8 bytes, and check for messages that aren't.

Pad T_PositionMsg to a multiple of 8 bytes, and check for messages that aren't.

T_PositionMsg had different sizes on 32 and 64 bit systems, which is bad when
a struct is put directly into an network message.

Try to work around this difference in old clients still on the network.