Optimize usage of SINGLE_HASH_MIN

[solardiz]

As @AlekseyCherepanov noticed, running "single crack" mode against bcrypt hashes on CPU without OpenMP is wasteful because our bcrypt implementation computes 3 hashes at a time, whereas SINGLE_HASH_MIN is 8, which isn't a multiple of 3. We should probably modify the code such that it rounds to a multiple of min_keys_per_crypt.

While looking into this, I see that for logging from john.c we try and infer the chunk size using SINGLE_HASH_MIN, but the actual code in single.c is now more complicated. So we may get logging inconsistent with what we actually do next. Fixing the above issue could introduce yet another inconsistency of this sort. In particular this message might be wrong:

        if ((options.flags & (FLG_SINGLE_CHK | FLG_BATCH_CHK)) && chunk < SINGLE_HASH_MIN)
                chunk = SINGLE_HASH_MIN;
        if (chunk > 1)
                log_event("- Candidate passwords %s be buffered and tried in chunks of %d",
                        min_chunk > 1 ? "will" : "may",
                        chunk);