This modifies the blockchain.ProcessBlock function to return an
additional boolean as the first parameter which indicates whether or not
the block ended up on the main chain.

This is primarily useful for upcoming test code that needs to be able to
tell the difference between a block accepted to a side chain and a block
that either extends the main chain or causes a reorganize that causes it
to become the main chain.  However, it is also useful for the addblock
utility since it allows a better error in the case a file with out of
order blocks is provided.