-Codechange: use IsValidXXX where ever possible
Note: both changes to prepare for new pool system, which needs those changes.
For every pool there are 2 ugly lines, which will be removed when done
implementing new pool system.
Based on FS#13 by blathijs, partly implemented.
The game date subsystem now allows someone to start in the year 0 and continue
up to the year 5 000 000. However, you currently cannot build anything before
1920 as there is no newgrf support for dates before 1920 or after 2090 yet.
-Cleanup: fix whitespace related coding style issues in date.[ch]
-Cleanup: make original comments doxygen compatible and remove/change outdated comments