RFC/Windows: look for USERPROFILE/HOMEPATH+HOMEDRIVE if HOME not defined?

Hello!

It occurs to me that HOME is not a standard environment variable
on Windows. This makes me think that perhaps, if not found on Windows,
we should be looking at more standard environment variables.
The easiest would be to look at USERPROFILE. Less easy, but probably
more correct, we could use %HOMEDRIVE% + %HOMEPATH%.

Any thoughts on that? If people agree, I could probably whip something
up! I think it would make GDB a little easier to use on Windows.

-- 
Joel