nsIEnvironment in Firefox 3.6a1pre

 Check the existence of an environment variable.
 This method checks whether an environment variable is present in
 the environment or not.

 - For Unix/Linux platforms we follow the Unix definition:
 An environment variable exists when |getenv()| returns a non-NULL value.
 An environment variable does not exist when |getenv()| returns NULL.
 - For non-Unix/Linux platforms we have to fall back to a 
 "portable" definition (which is incorrect for Unix/Linux!!!!)
 which simply checks whether the string returned by |Get()| is empty
 or not.

 @param aName   the variable name to probe.
 @return        if the variable has been set, the value returned is
                PR_TRUE. If the variable was not defined in the
                environment PR_FALSE will be returned.

 Get the value of an environment variable.

 @param aName   the variable name to retrieve.
 @return        returns the value of the env variable. An empty string
                will be returned when the env variable does not exist or
                when the value itself is an empty string - please use
                |exists()| to probe whether the env variable exists
                or not.

 Set the value of an environment variable.

 @param aName   the variable name to set.
 @param aValue  the value to set.