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)
qt: Use fixed-point arithmetic in amount spinbox

Fixes various issues and cleans up code

- Fixes issue #4500: Amount widget +/- has floating point rounding artifacts
- Amount box can now be emptied again, without clearing to 0

Also aligns the amount to the right, as in other places.
qt: Remove unused functions from BitcoinUnits

Remove two functions that are now unused.