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

First and foremost, this defaults to OFF.

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.
qt: Make error message for failed export a little friendlier

Closes #4528.
Merge pull request #3939

3da434a Introduce option to disable relay/mining of bare multisig scripts in TX outputs (Jeff Garzik)
Reuse sighash computations across evaluation