Make pool string->id operations properly length-aware

Make pool string->id operations properly length-aware

- Allow looking up and inserting partial key strings, this is useful
  in various cases where previously a local copy was needed for
  \0-terminating the key in the caller.
- Take advantage of rstrlenhash() in rpmstrPoolId(), previously the
  length was only interesting when adding so we wasted a strlen()
  on every call when the string was already in the pool.