[krbdev.mit.edu #2636] replace all calls to getenv()/setenv() with Get/SetEnvironmentVariable on Windows

[Jeffrey Altman via RT]

The getenv()/setenv() functions are local to the instance of the C
Runtime Library used by the library.  Changes to the process environment
made by the process after initialization will not be reflected in the
response.
This means that an app cannot change the default ccache by changing the
environment variable; or set/unset other options.