(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

(svn r3565) - Fix: On loading a game, GetPlayerRailtypes() didn't account for the fact that vehicles are introduced a year after their introduction date. This will also relieve possible (rare) network desyncs.

(svn r3564) Several smaller changes:
- Don't treat non-booleans as booleans
- Reduce variable scope
- Bracing
- Use DeMorgan's law to make conditionals easier to read
- if cascade -> switch
- Replace some magic numbers by symbolic names
- Avoid assignments within other statements

(svn r3563) Simplify the code for calculating the highscore table in network games and saving of old ecenomy data

(svn r3562) Remove pointless buffer copies

(svn r3561) Don't use FindLandscapeHeightByTile() when it's overkill. Also use a sprite enum instead of a magic number.