Check and revise monster power algorithm

[magnate]

While working on r1415 I thought it would be good to list visible monsters in descending order of power, regardless of depth. This immediately revealed a need to take a good look at the current monster power calculation, because the ordering made no sense whatsoever:

snaga (power 28)
uruk (power 24)
orc shaman (power 4)

So the snaga's power needs fixing at least, and possibly others. I'll start by printing out everything's power and having a look for outliers.