Change: replace per-AI "start_date" with a global "competitors_interval" (#10653)

The per-AI "start_date" is a lot of custom code, and was rarely
used in the way it was meant.

While at it, also ported this part over to the new timer system.

Remove: ENABLE_NETWORK switch

This switch has been a pain for years. Often disabling broke
compilation, as no developer compiles OpenTTD without, neither do
any of our official binaries.

Additionaly, it has grown so hugely in our codebase, that it
clearly shows that the current solution was a poor one. 350+
instances of "#ifdef ENABLE_NETWORK" were in the code, of which
only ~30 in the networking code itself. The rest were all around
the code to do the right thing, from GUI to NewGRF.

A more proper solution would be to stub all the functions, and
make sure the rest of the code can simply assume network is
available. This was also partially done, and most variables were
correct if networking was disabled. Despite that, often the #ifdefs
were still used.

With the recent removal of DOS, there is also no platform anymore
which we support where networking isn't working out-of-the-box.

All in all, it is time to remove the ENABLE_NETWORK switch. No
replacement is planned, but if you feel we really need this option,
we welcome any Pull Request which implements this in a way that
doesn't crawl through the code like this diff shows we used to.

Change: Allow AI companies to start immediately.

Allow multiple AIs to possibly start in the same tick.
start_date = 0 becomes a special case, where random deviation does not occur.
If start_date was not already 0, then a minimum value of 1 must apply.

(svn r18781) -Codechange: pass the CommandCost to the callback instead of whether it succeeded or not.
-Fix: AIs did update their last cost incorrectly in network games if the cost of the DC_EXEC phase differed from the ~DC_EXEC phase.

(svn r15077) -Codechange: enumify DAYS_IN_YEAR and DAYS_IN_LEAP_YEAR
-Change: when computing daily running cost, divide by 365 (instead of 364). Since r12134, the rounding errors don't need this correction anymore

(svn r15063) -Fix [NoAI]: starting companies now listen correctly to 'start_date' set to the AI slot (Yexo)
-Add [NoAI]: add a 'deviation' value for all settings, giving a slight deviation of the value of a setting (Yexo)

(svn r15045) -Add [NoAI API CHANGE]: in info.nut you can now have (optional) a CanLoadFromVersion(version), which should return true/false, to indicate if you can load a savegame made with your AI of version 'version'
-Add [NoAI API CHANGE]: in main.nut the Load() function now should be Load(version, data), where 'version' is the version of your AI which made the savegame
-Codechange [NoAI]: various of function renames to make things more sane
-Add [NoAI]: push the 'version' of the AI through various of layers
-Codechange [NoAI]: various of code cleanups
-Add [NoAI]: store the version of the AI in the savegame too

(svn r15027) -Merge: tomatos and bananas left to be, here is NoAI for all to see.
NoAI is an API (a framework) to build your own AIs in. See:
http://wiki.openttd.org/wiki/index.php/AI:Main_Page
With many thanks to:
- glx and Rubidium for their syncing, feedback and hard work
- Yexo for his feedback, patches, and AIs which tested the system very deep
- Morloth for his feedback and patches
- TJIP for hosting a challenge which kept NoAI on track
- All AI authors for testing our AI API, and all other people who helped in one way or another
-Remove: all old AIs and their cheats/hacks