GetEnv function

  DECLARATION:  GetEnv(EnvVar : String)

      PURPOSE:  Returns the value of an environment variable

         UNIT:  DOS

  RESULT TYPE:  String

      REMARKS:  The variable name can be in upper or lower case, but it
                may not include the equal sign (=) character.  If the
                specified variable does not exist, GetEnv returns an
                empty string.