Make SCRIPT_ENABLE_SIGHASH_FORKID a mandatory flag.

Summary: As per title.

Test Plan:

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

Reviewers: freetrader, CCulianu, #bitcoin_abc

Reviewed By: CCulianu, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D456