Pull a private hash-implementation copy to string pool

Pull a private hash-implementation copy to string pool

- The string pool is more specialized a data structure to be efficiently
  handled with the generic hash table implementation in rpmhash.[CH]
  and really requires quite a different approach.
- For starters, import a private copy generated roughly with:
      gcc -E -DHASHTYPE=poolHash \
             -DHTKEYTYPE="const char *" -DHTDATATYPE=rpmsid rpmhash.C
  ...and clean it up a bit: eliminate unused functions (except for
  stats which we'll want to keep for debug purposes), make remaining
  functions static and overall tidy up from the mess 'gcc -E' created.
  Lots of redundant fluff here still, to be cleaned up gradually...
- This doesn't change anything at all, but opens up the playground
  for tuning the pool hash implementation in ways the generic version
  could not (at least sanely) be.