(svn r3580) Some small cleanups before the real change, especially reduce the indentation by one by using continue

(svn r3579) v->u.rail.first_engine holds an engine type, not a vehicle index, so use INVALID_ENGINE.

(svn r3576) - Allow unused wagons have their ->first set. This fixes the faulty cache warning message, and noticably speeds up depot operations in large games.

(svn r3574) - [Merge] [GPMI] -Fix: finally OTTD doesn't compile if you do a 'make mrproper' (TrueLight)

(svn r3573) - Replace assert(0) with NOT_REACHED(). This commit sponsored by "giving Darkvater credit for the last three".

(svn r3572) - Rewrite GetFreeUnitNumber() so that only one loop of vehicles is required. Instead a list of used/unused numbers is created and the first unused number is chosen. This significantly improves performance in large games.

(svn r3571) - When loading a game, call TrainConsistChanged() for each train head separately before updating images, as v->first is used extensively in GetTrainImage() for custom graphics. This gives a significant speed improvement on loading a game.

(svn r3570) - Explicitly update v->first in TrainConsistChanged() if necessary, as this is far faster than brute forcing it later.

(svn r3568) - Fix: constantizing side-effect of qsort in r3563. Ugly? Yes. Makes MSVC happy? Yes.

(svn r3567) -Fix: for once and for all, use 'unsigned long' if you want to cast something
from int-like to void *... we fixed that not too long ago, and the code below
this changed line clearly tells you do to that :p