refactor(mhfpacket): rename 15 Unk fields with identified meanings

Replace unknown field names with descriptive names based on handler
logic analysis, switch dispatch patterns, DB query context, and
inline comments:

- ObjectHandleID, IsQuest, ItemIDCount, MaxCount, TokenLength,
  FormatVersion, LogoutType (high confidence from comments/constants)
- QueryType, DataType, MissionIndex, CheckOnly, RequestType,
  ExchangeType, TournamentID (confirmed by handler switch/if usage)

Also fix MsgSysLogout.Build calling ReadUint8 instead of WriteUint8.

server/channelserver/handlers_register.go

@@ -9,7 +9,7 @@ func handleMsgMhfRegisterEvent(s *Session, p mhfpacket.MHFPacket) {
 	pkt := p.(*mhfpacket.MsgMhfRegisterEvent)
 	bf := byteframe.NewByteFrame()
 	// Some kind of check if there's already a session
-	if pkt.Unk1 && s.server.getRaviSemaphore() == nil {
+	if pkt.CheckOnly && s.server.getRaviSemaphore() == nil {
 		doAckSimpleSucceed(s, pkt.AckHandle, make([]byte, 4))
 		return
 	}