Change rpmdb internals and iterator APIs to use the appropriate tag types

Change rpmdb internals and iterator APIs to use the appropriate tag types
- The actual indexes are rpmDbiTags, and the tag passed to
  rpmtsInitIterator() and rpmdbInitIterator() always refers to a
  database index, not arbitrary header tag. The tag passed to
  rpmdbSetIteratorRE() however /does/ refer to a header tag. This rather
  important distinction can actually be expressed in the API.
- As the indexes are generated from header tags of the same value,
  most of these need to use the TagVal types instead of actual enums
  to avoid mismatches - the value is what counts, not it being an enum.
- Several places still have variables misleadingly named "rpmtag" when
  they refer to dbi tags, but leaving them for now to avoid excessive
  changes at once.