move MANDATORY_SCRIPT_VERIFY_FLAGS to policy.h

Summary:
It really doesn't belong in standard.h, which entirely deals with the
concept of examining scriptPubKeys and determining what kind they are.
And in fact, the only direct usages of this flagset are immediately
below the new location:

1. as a baseline for STANDARD_SCRIPT_VERIFY_FLAGS, and
2. as a subtrahend for STANDARD_NOT_MANDATORY_VERIFY_FLAGS.

Test Plan: ninja all check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4926