Make multisig txns with non-null dummy nonstandard.

This commit helps prevent transaction malleability by enforcing that the
extra dummy value on multisig transaction script contains no data for a
transaction .  This syncs with a recent change in Bitcoin Core to remain
compatible.

As part of this change a new constant has been introduced which is used to
specify the script flags which are used for standard transactions.  This
constant is then used in both the memory pool and the mining code to
ensure they remain in sync with one another.

Closes #131.

ok @jrick, @dajohi