Reverting change to checkpoint validation

Apparently Warren already fixed this issue the right way!
Checkpoints will now be validated against known hashes again.
Fixing checkpoints

We need to use a null value when checking for difficulty transitions
as an indicator that we're syncing from a checkpoint.  This is the
only way to tell in this part of the code.  If we're not using a checkpoint,
the transition base (back an interval) will be non-null.  In the case
of the genesis block, we use interval - 1.
Litecoin: 0.8.6.1 reduced mininput from 0.0001 to 0.00001
Handle URI scheme
Fix missing feePerKb
Litecoin: adding dumpedprivatekey version to params