(svn r2448) General cleanup of rail related code, more to follow.

* Add: rail.[ch] for rail-related enums and wrapper functions.
* Codechange: Removed dozens of magic numbers with below enums.
* Codechange: Rewrote CheckTrackCombination().
* Add: TILE_SIZE, TILE_PIXELS and TILE_HEIGHT constants.
* Add: enums RailTileType, RailTileSubtype, SignalType to mask against the map arrays.
* Add: enums Track, TrackBits, Trackdir, TrackdirBits for railway track data. (Note that the old RAIL_BIT constants are replaced by TRACK_BIT ones).
* Add: enums Direction and DiagDirection
* Codechange: Moved a bunch of track(dir) related lookup arrays from npf.[ch] to rail.[ch].
* Codechange: move RailType enum from tile.h to rail.h.
* Add: Wrapper functions for masking signal status in the map arrays: SignalAlongTrackdir, SignalAgainstTrackdir and SignalOnTrack.
* Add: Wrapper functions to access rail tiles, using above enums
* Add: Wrapper functions to modify tracks, trackdirs, directions, etc.
* Add: Wrapper functions for all lookup arrays in rail.[ch] (Arrays are still used in parts of the code)
* Codechange: Renamed some variables and arguments to better represent what they contain (railbit -> track, bits -> trackdirbits, etc.). 
* Codechange: Don't use FindLandscapeHeight() in CmdRemoveSingleRail(), since it returns way too much info. Use GetTileSlope() instead.
* Codechange: [NPF] Removed some unused globals and code from npf.c.

tile.h

@@ -18,17 +18,30 @@ typedef enum TileType {
 	MP_UNMOVABLE
 } TileType;
 
-/* XXX: This should be moved out to a new file (rail.h) along with some other
- * cleanups. I'll do that after 0.4) */
-typedef enum {
-	RAILTYPE_RAIL   = 0,
-	RAILTYPE_MONO   = 1,
-	RAILTYPE_MAGLEV = 2,
-	RAILTYPE_END,
-	RAILTYPE_MASK   = 0x3,
-	INVALID_RAILTYPE = 0xFF,
-} RailType;
+/* TODO: Find out values */
+/* Direction as commonly used in v->direction, 8 way. */
+typedef enum Directions {
+	DIR_N   = 0,
+	DIR_NE  = 1,      /* Northeast, upper right on your monitor */
+	DIR_E   = 2,
+	DIR_SE  = 3,
+	DIR_S   = 4,
+	DIR_SW  = 5,
+	DIR_W   = 6,
+	DIR_NW  = 7,
+	DIR_END,
+	INVALID_DIR = 0xFF,
+} Direction;
 
+/* Direction commonly used as the direction of entering and leaving tiles, 4-way */
+typedef enum DiagonalDirections {
+	DIAGDIR_NE  = 0,      /* Northeast, upper right on your monitor */
+	DIAGDIR_SE  = 1,
+	DIAGDIR_SW  = 2,
+	DIAGDIR_NW  = 3,
+	DIAGDIR_END,
+	INVALID_DIAGDIR = 0xFF,
+} DiagDirection;
 
 void SetMapExtraBits(TileIndex tile, byte flags);
 uint GetMapExtraBits(TileIndex tile);