(svn r2290) - CodeChange: protect the next batch of commands. This brings us to a total of 61, which is 53% :)

- CodeChange: To correctly accept engine-prototypes, the best-player checking has been moved to its own function, I hope it functions the same as before.
- CodeChange: Added symbolic types of PlayerID, OrderID and EngineID. For engines also added GetEngine() and IsEngineIndex(), similar to the other such functions.
- CodeChange: To correctly build industries, some tables have been moved to build_industry.h. The only way to find out currently if an industry is valid in a climate is by looping all industries and checking if it matches. Also to comply with the patch setting build_rawmaterial_industries, it is assumed that these industries do not accept any cargo of any type. This can and probably should changed in the future to some flag in their struct. Also use _opt_ptr instead of _opt.
- CodeChange: implemented the HQ checking code inspired by MarkR2 in "[ 1190944 ] Many commands not checked for security".  Unfortunately it is impossible to prevent only deleting a HQ by a modified client atm.
- CodeChange: For insert order and modify order their parameters are implicitely truncated to 8 bits, instead of the 16 bits said in the comments.

command.c

@@ -208,7 +208,6 @@ static CommandProc * const _command_proc_table[] = {
 	CmdTrainGotoDepot,						/* 39 */
 	CmdForceTrainProceed,					/* 40 */
 	CmdReverseTrainDirection,			/* 41 */
-	/***************************************************/
 
 	CmdModifyOrder,								/* 42 */
 	CmdSkipOrder,									/* 43 */
@@ -237,6 +236,7 @@ static CommandProc * const _command_proc_table[] = {
 
 	CmdSellAircraft,							/* 59 */
 	CmdStartStopAircraft,					/* 60 */
+	/***************************************************/
 	CmdBuildAircraft,							/* 61 */
 	CmdSendAircraftToHangar,			/* 62 */
 	CmdChangeAircraftServiceInt,	/* 63 */
@@ -302,7 +302,7 @@ static CommandProc * const _command_proc_table[] = {
 	CmdStartScenario,							/* 109 */
 	CmdBuildSignalTrack,					/* 110 */
 	CmdRemoveSignalTrack,					/* 111 */
-	CmdDestroyCompanyHQ,					/* 112 */
+	CmdDestroyCompanyHQ,					/* 112    <-- done */
 	CmdGiveMoney,									/* 113 */
 	CmdChangePatchSetting,				/* 114 */
 	CmdReplaceVehicle,						/* 115 */