Add all active script flags where appropriate

[tyler-smith]

At least two places introduced in PR #59:

bchd/mining/mining.go

Line 480 in 17e1dea

var scriptFlags txscript.ScriptFlags

bchd/blockchain/validate.go

Line 588 in 17e1dea

var scriptFlags txscript.ScriptFlags

This works as expected currently because none of the downstream users branch based on any other flag but this might not always be true. We should make these the full set of active flags.

Part of this is probably abstracting the instantiation of these flag sets based on the current environment.