1. HVSTRLEN -> HSTRLEN. It's unlikely one needs the length of the key,
   not clear how the API would work (by value does not make sense) and
   there will be better names anyway.
2. Default is to return 0 when field is missing.
3. Default is to return 0 when key is missing.
4. The implementation was slower than needed, and produced unnecessary COW.

Related issue #2415.