Target subtypes must now implement another repository hook to validate
any CredentialLibraries that are being added/set. This allows for
subtypes to determine the valid set of CredentialLibraries.
For tcp.Targets only CredentialLibaries with
credential.ApplicationPurpose are allowed. If any other purpose is
provided an error is returned.
This moves all of the tcp target functionality into a tcp package. It
also establishes a Register function in the target package to allow
specific target types to be registered at initilazation time. When
registering, a target type needs to provide its Subtype, as well as some
RepositoryHooks that are used by the target Repository.
Timothy Messier