Unify the user+group caching between librpm and librpmbuild

Unify the user+group caching between librpm and librpmbuild
- The build version has getUidS() and getGidS() for which there is
  no counterpart in the rpmug api but there's not much point to them:
  they check whether the user/groupname exists but return our own
  string back to us even if it doesn't.
- The build version also caches more than current rpmug, but has an ugly
  hardwired limit causing (in theory) errors that can't be nicely
  handled, and is the last piece relying on RPMLOG_CRIT actually
  terminating the process. The librpm version doesn't fail, in the
  worst case its just a bit slower. And that can be fixed anytime by
  making it to use hash tables for caching.