make D windows-aware

[Jarrett Billingsley]

DMD is Windows-aware in the sense that if you have a WinMain(), it emits a 
reference to acrtused or whatever, but what I mean is make it so we don't 
have to manually put all the D init / deinit code in the WinMain().  I mean, 
when we define main() for a console program, it automatically calls dmain() 
which calls main().  Why not make it so when we define WinMain(), it 
automatically calls DWinMain() before our WinMain() or something?

I can only see this as an advantage, as well, as then any fancy things 
Walter puts into dmain (like exception stack traces and just exception 
handling in general) will be put into DWinMain as well, instead of the 
"WinMain" living in (of all places) the docs and having to be copied into 
any Windows programs.