[secp256k1] Backport "'Schnorrsig API overhaul' fixups"

Summary:
Bitcoin Core added some later fixups, which make sense to be backported after we added D16958.

This is a backport of secp256k1#963.

Test Plan: ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16968