restore divided hashrate for quark and jackpot

The previous way it was done was not correct (hashdone divided/2)

So add a factor 0.5 to the computed hashrate... to stay comparable
api: add histo command and difficulty

enhance multi-gpu stats and fix nvapi indexes

change syslog prefix to ccminer (cpuminer remains)

api 1.1 modified - not officially released yet
nvapi: fix mapping of devices