Sanitize rpmVerifySignature() a bit

Sanitize rpmVerifySignature() a bit

- Hash context is required for everything, require non-NULL ctx
  in rpmVerifySignature() already
- pgpDig is only relevant for true signature, digest checking doesn't
  need it - dont require dummy dig to be passed for digests.
- Treat unknown signatures as a case of bad parameters: we're the
  only caller of rpmVerifySignature() so it'd be us screwing up if
  we ask for unknown signature to be verified.
- Treat bad parameters as a hard failure instead of "not found",
  bad parameters mean we cannot verify the signature which really
  equals FAIL.