Add SCRIPT_ALLOW_SEGWIT_RECOVERY
Summary: Flag that allows the spending of p2sh segwit coins
Test Plan: test_bitcoin --run_test=script_tests
Reviewers: deadalnix, O1 Bitcoin ABC, #bitcoin_abc
Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc
Subscribers: markblundeberg, teamcity, schancel
Differential Revision: https://reviews.bitcoinabc.org/D2468