Introduce rpm tag "classes"

Introduce rpm tag "classes"
- rpm tag data can be either numeric, strings or binary data, each with
  their own "subclasses" (different sized integers etc), add new
  enumeration for these
- add rpmTagGetClass(), rpmtdClass() public functions for retrieving the
  base class of tag and container
- useful for getting a basic idea how to handle tag/container data