[phobos] getenv, setenv and unsetenv implemented for Windows

[Walter Bright]

David Simcha wrote:
> Just as a general reference since I've been meaning to ask for awhile 
> and this reminded me, do we really still care about supporting 
> '9x/ME?  The last version in this line was released an entire decade 
> ago and hasn't been supported by Microsoft in 4 years.  Also, 
> according to this link (http://www.w3counter.com/globalstats.php) none 
> of the '9x/ME versions of Windows even register on the market share 
> meter anymore.  I'm not saying we should go out of our way to cripple 
> '9x/ME support, but I certainly don't think it's worth any serious effort.
>

No, I don't think we really care, but in many cases it's trivial to 
support it (like in std.file), and so why not?