Push RPMDBI_LABEL arch parsing down to rpmdb layer to fix stuff

Push RPMDBI_LABEL arch parsing down to rpmdb layer to fix stuff

- Partial NEVRA labels cannot be reliably parsed, the various combinations
  need to be figured out by trial-and-error. The rpmts layer doesn't stand
  a chance of getting it right so move it to rpmdb layer. This doesn't
  make the process any less stupid, but at least we get correct results...
- Fixes iterator count when arch is used in a label and more than one
  arch variants of a package are installed. Previously iterator count
  could be more than one despite actual iteration only hitting one
  match, as the arch RE match was added after already initializing
  the iterator.
- Also fixes various pathological cases:
  - If a legal arch was part of name, version or release (stupid but legal)
    we misinterpreted it for arch and failed to find the package.
  - If a package with unknown architecture was installed (with --ignorearch)
    we could not remove it by its arch as we relied on rpmIsKnownArch()