Mask mode: Support -min-len.

Fixes #380.

@Sayantan2048 please review. Can we merge this or do you have a better solution?

Now, we can eg. exhaust LM like this:
```
../run/john test.sam -format:lm -1=?u?d?s -mask=?1?1?1?1?1?1?1 -min-len=0
```
do_mask_crack() will actually iterate over all lengths between 0 and 7 (which is the format's max length) and call a new back-end mask_crack() function.