[RFA/mingw32] environment variables are case-insensitive on win32

[Jerome Guitton]

Jerome Guitton (guitton@adacore.com):

> > BTW, this would be more correctly modeled as a target property, not
> > host property.
> 
> Hm, indeed. I'll add a new hook in gdbarch then.

Hmm. In theory that would be true, but the problem is that as of today
the environment seems to come from the host: in each inferior record, the
environment field is initialized by make_environment, which gets
it from the host. I cannot find any place where this would be updated
by gdbarch.

Actually, the semantics of the "path" command is not quite clear to me
when host != target.  In particular, you usually call it before
creating the inferior...