* Add search terms to command
* Rename Signature desc to usage
To be named uniformly with extra_usage
* Throw in foldl search term for reduce
* Add missing usage to post
* Add search terms to signature
* Try to add capnp Signature serialization
This restores a basic version of the `post` command.
Some source types have been omitted from this first take.
I copied from `fetch` and from `post`@0.40.0.
Part of #4356
