bug: Byzantine variant inconsistencies

[meling]

• The cue configuration specifies slow as a variant, but there is no implementation. Not sure what I had in mind, or if this is even due to me...
• The cue configuration does not accept fork and increaseview for which we do have an implementations.
• The internal/cli/root.go allows to specify --byzantine-strategy, but only accepts silentproposer and fork.