checkmultisig: refactor nullfail check

Summary:
For the schnorr mode, this check is meaningless since fSuccess=true
always, so move it into legacy mode.

Also, make available the allsigsnull value for the sake of SigChecks
counting in a future Diff.

Test Plan: make check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Maniphest Tasks: T704

Differential Revision: https://reviews.bitcoinabc.org/D4677