[sieve] clarify and fix backward compatibility for index extension

bc_eval.c:
- pass version to eval_bc_test() for backward compatibility

sieved.c:
- pass version to dump2_test() for backward compatibility

bc_eval.c and sieved.c:
- BC_ADDRESS_PRE_INDEX: convert bytecode to host format prior to using
- ADDRESS_PRE_INDEX and HEADER_PRE_INDEX: break before default case
- BC_DATE and BC_CURRENTDATE:
  - use the heuristics only for the specific ambiguous version of the
    bytecode
  - clarify how the heuristics work
  - resolve the ambiguous case based on the version of bytecode that
    was actually in Kolab Groupware 3.3