Truncate pilot name at null
Make kill_matrix uint16_t, not short
Pass net_udp buffers as const
Throw on net_udp_read_sync_packet failure

Callers expect sane Player_num, but on failure, it is invalid.
Returning a failure status causes net_udp_level_sync to fault using
Players[Player_num].connected, so throw an exception instead.
Fix player clear range
Refactor callsign_t constants
reenable Int3 debugging
check for GM_MULTI before trying to do network jobs