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