completions: Fix completion for `kbsecret help`.
completions: Fix completion for `kbsecret help`.

The completion routines currently aren't smart enough to work with
builtins, so `kbsecret help` needs to be handled at the toplevel.

kbsecret/kbsecret,woodruffw/kbsecret,kbsecret/kbsecret