(svn r6058) -Fix: Get(Industry|Town)ArraySize could never return 0
Note: _total_towns and _total_industries willb e removed soon, so this 'hack' is okay, for now ;)

(svn r6057) -Codechange: made a function GetRandomXXX, that _always_ returns a valid XXX, unless there are none to pick from. Then NULL is returned.

(svn r6056) -Codechange: don't use a loop what can be done with a simple wrapper

(svn r6055) -Codechange: added GetXXXArraySize, which returns HighestID + 1 (or, will do that).
It isn't the best name, but we couldn't find any better.
This unifies the pool-system even more.

(svn r6054) Change the sign sorter in a similar way as the other sorters: Remember a list of pointers to signs instead of a list of SignIDs - This removes a layer of indirection
Also make the sign list static

(svn r6053) -Codechange: renamed all IsXXXIndex to IsValidXXXID
-Codechange: IsValidXXXID now also checks if XXX is really valid, not if the number is within range
Both changes again in preperation of the new mem-pool system, which requires this.
IsValidXXXID is not a bit less pretty, but that will be cleaned up after the new mem-pool system

(svn r6052) -Codechange: change OrderType (order->type) in a typedef
-Codechange: renamed DeleteDestinationFromVehicleOrder to RemoveOrderFromAllVehicles to reflect his function better
-Codechange: changed the params of RemoveOrderFromAllVehicles, to avoid unneeded variable-creation

(svn r6051) -Fix r6047: forgot many times a ! :(

(svn r6050) -Codechange: mass-renamed SignStruct -> Sign and ss -> si. Now functions and variables all match eachother

(svn r6049) -Codechange: forgot EngineRenew in r6047
-Codechange: cleaned up the EngineRenew code a bit (coding style mostly)
-Codechange: forgot the correct comment in station_cmd
-Codechange: move pool-stuff to engine.h, like we always do