util: Work around broken Scrypt pools automatically using realistic pdiff assumptions

Reasoning: we need to add another condition to the existing "if (opt_scrypt)" work-around as ASICs approach pdiff 1.
If we are to add an additional check for pdiff > N, it makes sense to use a realistic value to adjust all Scrypt pool diffs.
Merge pull request #516 from nwoolls/feature/broken-scrypt-diff-auto

util: Work around broken Scrypt pools using realistic pdiff assumptions