Reduced threshold Segwit MASF.

Sets out how miners could push segwit through with about 80% support, instead of the 95% it originally needed.

Once about 80% of recent blocks signal for this BIP, any block that does not also signal for segwit is rejected. That pushes segwit's own vote past its 95% threshold. The rule stops applying once segwit is locked in, active or failed.