Merge zPiv fixes into GambleCoin

[CaveSpectre11]

Merge in the Pivx zPiv vulnerability fixes into GambleCoin, so that we can re-enable zPiv support

[CaveSpectre11]

This is on hold indefinitely given the libzerocoin vulnerability that has surfaced. This may be revisited in order to allow people to unmint their zGMCN back to GMCN. For the time being; the zerocoin spork will remain in effect, and v1.2 will disable the spork and make the feature permanently disabled.