Enhanced string hash to permit calculating string length on the same call

Enhanced string hash to permit calculating string length on the same call

- String hashing needs to walk the entire string anyhow, might as well
  take advantage of this and have it return the string length to avoid
  having to separately call strlen() in the cases where this matters.
- Move the implementation into rpmstrpool.c for inlining possibilities,
  rstrhash() is now just a wrapper to rstrlenhash(). The generic
  hash implementation could not take advantage of this anyway really.