Uppercase only the PATH env variable

[claudiubelu]

busybox's env behaves differently, depending on how it is
called:

busybox.exe env # env variable names casing is preserved.

or

busybox.exe sh -c "env" # env variable names are uppercased.

But env on Linux preserves the casing, and so should busybox,
with the exception of the PATH variable, which must be uppercased,
otherwise the default value will be used instead, which can lead to
commands not being found.

[rmyorston]

It isn't env that's behaving differently, it's the shell. env always preserves the case of environment variables.

When the busybox-w32 shell is started it loads the Windows environment variables into shell variables, converting their names to uppercase. When env is run from the shell the shell variables are placed in its environment.

I think converting the environment variable names to uppercase is the correct thing to do. (It's also how Python handles this issue.)