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
Enable Int3 as d_debugbreak

Change Int3 from a no-op to a platform-specific trap to debugger.

Requested by btb: https://github.com/dxx-rebirth/dxx-rebirth/pull/18
Merge commit 'Re-enable debug breaks' into unification/master

Requested by btb: https://github.com/dxx-rebirth/dxx-rebirth/pull/18
Fix off-by-one end causing memory corruption
Fix uninitialized variable in render reprocess
Stop briefing search on dot or null

Mission names are passed in without an extension.  Short mission names
read beyond the end of the buffer, causing AddressSanitizer to kill the
program.

Reported by btb: https://github.com/dxx-rebirth/dxx-rebirth/issues/20
Use partial_range on Triggers
Use partial_range when searching HighestLevels