util: Work around broken Scrypt pools 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.
Ya not >=, I should rename the const to be minimum_acceptable_scrypt_diff, the intent was to allow 64 to be a "real" Scrypt diff. See the comments above. I've been unable to find a pool that has anything lower than 128 for even CPU miners.