+ Improve the handling of 'friends' in the 'replace' command.  Enable a value to be replaced if it hasn't already been set in another package, or if it has been set previously in the currently package without being explicitly listed as a 'friend'.
+ Remove the unecessary checking for an existing value in the CSH
implementation of append and prepend.
+ Add 'replace' as a command to the EnvironmentVariable class.
+ Check for the key on self, as well as (the apparently unused)
self.environ.  This is necessary to make append and prepend work as
expected.