Merge #13429: Return the script type from Solver

Summary:
984d72ec659361d8c1a6f3c6864e839a807817a7 Return the script type from Solver (Ben Woosley)

Pull request description:

Because false is synonymous with TX_NONSTANDARD, this conveys the same
information and makes the handling explicitly based on script type,
simplifying each call site.

Prior to this change it was common for the return value to be ignored, or for the
return value and TX_NONSTANDARD to be redundantly handled.

Tree-SHA512: 31864f856b8cb75f4b782d12678070e8b1cfe9665c6f57cfb25e7ac8bcea8a22f9a78d7c8cf0101c841f2a612400666fb91798bffe88de856e98b873703b0965

Backport of Core PR13429 (Modified to fit our codebase)
https://github.com/bitcoin/bitcoin/pull/13429/

Test Plan:

make check

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4933