Simplify stratum to convert difficulty to target immediately
Rework difficulty-to-target functions to error on the safe side, and consistently work in pdiff or bdiff

Benchmark work and default stratum target are now pdiff 1 instead of bdiff 1
Stratum: Interpret integer difficulty values as pdiff (or Ldiff, for scrypt) to avoid losing shares on non-compliant servers
Stratum: If the pool target has changed since the job a work came from, check any shares against the new target too in case it expects retroactive difficulty adjustments
Stratum: If difficulty calculates out to 0.999 to 1, round it up to 1 to avoid something SHA2 hardware cannot handle
Merge branch 'stratum_target' into bfgminer

Conflicts:
	miner.c