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

[Ken Raeburn via RT]

> Unfortunately not.  GetEnvironmentVariable() requires that the user
> provide the input buffer as well as a length.  The return value is the
> length of the returned string.

On the plus side, that'll likely be a thread-safe interface, compared 
with the UNIX one (if setenv is used).  Perhaps we should modify our 
uses of getenv to call a helper function which returns us a string in 
allocated storage; then that helper function could use either getenv or 
GetEnvironmentVariable, and in the getenv case, while it still wouldn't 
be thread-safe, it would reduce the window of exposure somewhat.

Ken