Included a unit test for IsWitnessProgram, a few other test cases for ALLOW_SEGWIT_RECOVERY and created a wrapper for IsWitnessProgram()

Removed unused parameters from IsWitnessProgram

Refactored to be restrictive to segwit programs.

You can try it by yourself in mainnet:

Removed PreviousSpendableOutput() and changed to network_thread_start()

Did you notice bitcoind is still returning 'non-mandatory-script-verify-flag (Script did not clean its stack)' even though CLEANSTACK is mandatory right now? Should the recently activated flags be included in MANDATORY_SCRIPT_VERIFY_FLAGS, like you did previously?

Refactored next_block, removed useless test and included a test to verify whether the activation was latched during a reorg

Oh I see. My original intention was to converge on the simplest solution (spec, implementation and verification-wise) that allowed the retrieval of segwit coins while keeping all the malleability gains from the cleanstack rule. This proposal was not specific to segwit so the requirement description (which I still need to write and create a PR on github) and verification are relatively simple. Is it really your wish to be this specific on segwit? All those segwit specific requirements (e.g: redeem script size between 4 and 42, starting with OP_0, ..., OP_16, etc) will have to go into the spec for the HF and, as you pointed out, will require a few extra tests.

Simplified the changes in VerifyScript