Introduce option to disable relay/mining of bare multisig scripts in TX outputs

First and foremost, this defaults to OFF.

Bare Multisig in TX outputs (scriptPubKey) are increasingly being used for
raw data storage.  This option lets a node consider such transactions
non-standard, meaning they will not be relayed or mined by default, but
other miners are free to mine these as usual.

Suggested by Peter Todd.