r377: increased unpaired penalty from 9 to 17

This leads to more aggressive pairing - more properly paired reads. I have
found a few cases where, for example, read1 is umambiguously mapped to chr20
while its 100bp mate has a perfect match to another chr but has 3 mismatches
and 1 deletion when it is paired with read1 on chr20. With longer reads, it
seems that the chr20 hit is correct, although it is not obvious how this
happened in evolution.