(svn r6754) -Codechange: Remove some #idef ENABLE_NETWORK specific defines. With networking
disabled hardcode _networking/_network_available to zero and let the compiler
handle all optimizations.

(svn r6047) -Codechange: FOR_ALL now _only_ loops valid items, and skips invalid ones
-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.

(svn r5999) -Feature: change the original date format to a 32 bits format based at the year 0.
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.

(svn r5926) -Codechange: make _cur_year contain the full year, instead of the offset since 1920
-Codechange: store all year related variables that are _not_ stored in a savegame/transported over the network in the same format as _cur_year

(svn r5918) -Cleanup: rename ConvertDayToYMD/ConvertYMDToDay as they really convert a Date to/from a YearMonthDay.
-Cleanup: swap ConvertDateToYMD's parameters to make the order consistent with the name of the function.

(svn r5916) -Cleanup: use MIN_YEAR/MAX_YEAR for the year boundaries and BASE_YEAR when comparing _cur_year with a 'full' year.
-Cleanup: replace some magic '1920' values with BASE_YEAR.

(svn r5887) -Cleanup: move date related functions, defines and variables to date.[ch]
-Cleanup: fix whitespace related coding style issues in date.[ch]
-Cleanup: make original comments doxygen compatible and remove/change outdated comments