Differentiate between non-existent and invalid region tag

Differentiate between non-existent and invalid region tag

- Non-existent region tag is very different from existing but invalid
  one - the former is not an error but the latter one is, and needs
  to be handled as such. Previously an invalid region tag would cause
  us to treat it like rpm v3 package on entry, skipping all the region
  sanity checks and then crashing and burning later on when the immutable
  tag is fetched.
- Refer to REGION_TAG_TYPE instead of RPM_BIN_TYPE wrt the expected
  type of region tag for consistency and clarity, they are the same
  exact thing though.
- Should unify these damn copy-slop check one of these days, sigh...
  For now, settling for the easily backportable approach.
- Fixes the other half of CVE-2012-0060