Plugin for pubsub encryption and signing were named using prototype name as the
specifications where not yet published at the time. Since, those specifications have been
accepted, so we can rename those plugins using the corresponding XEP numbers.
Doc has been updated to remove old mention saying that the XEP was still being evaluated.
rel 475
- add a `Pubsub Signature` section to `encryption` documentation
- document `-X, --sign` flag where it's used
- document `pubsub/signature` subcommands
fix 381
