Document our supported rpmdb indexes in the API via an enum of their own

Document our supported rpmdb indexes in the API via an enum of their own
- Now that our indexes are actually hardwired in the code we can
  properly separate rpmtag vs db index names.
- Defined similarly to rpmTags, with rpmDbiTag being the "true enum"
  and rpmDbiTagVal the integral type they present. As many of the
  indexes are defined via RPMTAG values (and need to remain that
  way to avoid breaking every single API user) we need to have a
  compatible type with rpmTags.
- These would logically belong to rpmdb.h but keeping in rpmtag.h
  to have them available "everywhere", we can't go including rpmdb.h
  from rpmtag.h so keeping them here avoids potential unnecessary breakage.