Maintaining the storage layer/drivers in two languages is burdensome. Since the integration tests require the Perl storage layer/drivers we'll need them even after the core code is migrated to C. Create an interface layer so the Perl code can be removed and new storage drivers/features introduced without adding Perl equivalents.
The goal is to move the integration tests to C so this interface will eventually be removed. That being the case, the interface was designed for maximum compatibility to ease the transition. The result looks a bit hacky but we'll improve it as needed until it can be retired.
This was not enforced at parse time because repo1-cipher-type could be passed on the command-line even in cases where encryption was not needed by the subprocess.
Filter repo-cipher-type so it is never passed on the command line. If the subprocess does not have access to the passphrase then knowing the encryption type is useless anyway.
