(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.

Scripting
---------

OpenTTD supports scripts.

local scripts:
 - 'autoexec.scr' is executed on gamestart [all - use this for custom aliases per ex.]

+network scripts:
   should be used to set client optimization settings:
 - 'on_client.scr' is executed when you join a server [all clients]

   should be used to set the servers port/ip and/or server optimization settings/patches:
 - 'pre_server.scr' is executed before the servers tcp stack is started [in-game only]
 - 'pre_dedicated.scr' is executed before the servers tcp stack is started [dedicated only]

   should be used to set the servers name, password and so on:
 - 'on_server.scr' is executed after starting a server [dedicated and in-game]
 - 'on_dedicated.scr' is additionally executed after starting a server [dedicated only]

For examples how a script can look, check the .example examples.