Implement CBlockIndex::GetBlockHeader for auxpow block headers.  In this
case, load the header from disk.  Add a function to do that (as opposed
to reading the full block).

Update the regtest genesis block nonce to yield a valid block hash.  Fix
the resulting hash as checkpoint and with assert.

Not yet very well tested, and still missing getauxblock.  But it still
accepts the existing blockchain, which is a good start. :)