Resolve "Very Slow On Windows" issue

In response to thread https://github.com/lib/pq/issues/69.

lib/pq is using Go's user.Current() function to grab the current user name for the case where the user name has not been set by the "user" parameter or the respective environment variable.  Go's user.Current() performs the user name differently than the Win32 API call used in postgresql's libpq.  This change set alters the windows only user name fetch to be identical to that used in libpq.