Separate match iterator allocation vs initialization

Separate match iterator allocation vs initialization
- There's exactly one spot within rpm which needs the former behavior
  of rpmdbInitIterator() on a secondary index with keyp of NULL:
  rpmFindBaseNamesInDB(): it wants an empty iterator on RPMDBI_BASENAMES,
  which is then extended as it progresses. Starting with commit
  c70e076e088589f65160c05ee7cc8db0b3db6d7e, rpmdbInitIterator()
  however returns the actual index in that case, which is not
  optimal for rpmFindBaseNamesInDB(). Handle the basenames in db
  as the special case it is, and have it explicitly create an
  empty iterator with the new call.