make QProcessEnvironment on Windows preserve variable name case

make QProcessEnvironment on Windows preserve variable name case

while windows itself does not care which case the variable names are in,
they may be passed to unix tools which *do* care.

note that this uses true case folding for string comparisons while
windows uses uppercasing. this means that "ess" and "eß" will be
considered the same by us, while not by windows. this is not expected to
have real-world impact, particularly because non-ascii variable names
are not used much.

Task-number: QTCREATORBUG-3110
Reviewed-by: thiago
Reviewed-by: dt
(cherry picked from commit f3db5603871928ebed43a085a496397e65952b39)