Return fingerprint lookups through retval pointer, not struct

Return fingerprint lookups through retval pointer, not struct

- Returning structs by value is a bit icky, pass in a fp pointer
  for fpLookup() to fill in instead. This leaves the actual return code
  free for handling errors (but ignoring that for now as we always have)
  The other option would be always mallocing the return, and we dont
  want to do that...
- Shouldn't change any actual functionality.