clarify build.d documentation

Fix build.d target executable name on Windows.

Fix Windows binary releases

Use build.d everywhere and take arguments by parameter ...

It seems easier to forward arguments from the build script this way,
and it's much more common to change the build arguments than the version.

Replace `build.sh` and `build.cmd` with `build.d` ...

Since dub needs a D compiler anyway, we might follow in DMD's step and use D for building,
as it is more reliable and expressive than a shell script as features pile up.

This unifies the two scripts to expose the same set of functionalities on all platforms,
with a behavior that is hopefully more sensible
(tradeoff and reasoning are explained through the code / documentation).
It is also completely independent from the current working directory.

Some discrepancies included:
- Name of the variable to override to select the compiler (DC vs DMD);
- Version file: Windows was always regenerating it;
- Version file: Windows would display "unknown" if `dub describe` failed,
  POSIX would use an empty string;
- Compiler detection: POSIX would detect ldmd2 / gdmd if dmd is not present,
  Windows would just fail;

All those functionalities are now unified in a single script.
The old script are kept for compatibility, but forward to the new script,
and can be removed in a few releases.