Add LOW_S and NULLFAIL to mandatory flags
Summary: As per title. These are now mandatory.
Test Plan:
make check ../qa/pull-tester/rpc-tests.py
Do IBD
Reviewers: schancel, #bitcoin_abc, Mengerian
Reviewed By: #bitcoin_abc, Mengerian
Subscribers: Mengerian
Differential Revision: https://reviews.bitcoinabc.org/D768