Make SCRIPT_VERIFY_STRICTENC a mandatory flag

Summary: This flag has been made mandatory by the HF.

Test Plan:

make check
../qa/pull-tester/rpc-tests.py

Reviewers: freetrader, CCulianu, #bitcoin_abc, sickpig

Reviewed By: freetrader, #bitcoin_abc, sickpig

Differential Revision: https://reviews.bitcoinabc.org/D421