Merge #15934: Merge settings one place instead of five places

083c954b02a4e7d0708349eeaf3bac2b5947fb0e Add settings_tests (Russell Yanofsky)
7f40528cd50fc43ac0bd3e785de24d661adddb7a Deduplicate settings merge code (Russell Yanofsky)
9dcb952fe5f85529ab28e091af7534e72c21c90f Add util::Settings struct and helper functions. (Russell Yanofsky)
e2e37cfe8af088bd8ea884be2f79f0f3cac555d5 Remove includeconf nested scope (Russell Yanofsky)
5a84aa880f6da0bac0e2144733fdef3b8558c761 Rename includeconf variables for clarity (Russell Yanofsky)
dc8e1e75487461ec9bff433144f0db831b682403 Clarify emptyIncludeConf logic (Russell Yanofsky)

Pull request description:

  This is a refactoring-only change that makes it easier to add a new settings source.

  This PR doesn't change behavior. The [`util_ArgsMerge`](https://github.com/bitcoin/bitcoin/blob/deb2327b435925c6a39ca654a79283b8eb6aeb86/src/test/util_tests.cpp#L626-L822) and [`util_ChainMerge`](https://github.com/bitcoin/bitcoin/blob/deb2327b435925c6a39ca654a79283b8eb6aeb86/src/test/util_tests.cpp#L843-L924) tests added in #15869 and #15988 were written specifically to confirm that ArgsManager settings are parsed, merged, and returned the same way before and after this change.

  This change:

  - Makes it easier to add new settings sources that can get merged with existing sources (see 70675c3e4975203ad6222ba2b00c83b4e4213793 from #15935).
  - Separates parsing of settings from merging of settings, and deduplicates merging code so it doesn't happen five different places ([GetArg](https://github.com/bitcoin/bitcoin/blob/c459c5f70176928adcee4935813a2dbe7f4dbd51/src/util/system.cpp#L221-L244), [GetNetBoolArg](https://github.com/bitcoin/bitcoin/blob/c459c5f70176928adcee4935813a2dbe7f4dbd51/src/util/system.cpp#L255-L261), [GetArgs](https://github.com/bitcoin/bitcoin/blob/c459c5f70176928adcee4935813a2dbe7f4dbd51/src/util/system.cpp#L460-L467), [IsArgNegated](https://github.com/bitcoin/bitcoin/blob/c459c5f70176928adcee4935813a2dbe7f4dbd51/src/util/system.cpp#L482-L491), [GetUnsuitableSectionOnlyArgs](https://github.com/bitcoin/bitcoin/blob/c459c5f70176928adcee4935813a2dbe7f4dbd51/src/util/system.cpp#L343-L352)) in inconsistent ways.
  - Documents and tests current strange merging behaviors, so they be cleaned up in the future if resulting code simplifications and UX improvements warrant loss of backwards compatibility. The newly documented behaviors are: command line [ignored arguments](https://github.com/ryanofsky/bitcoin/blob/69d44f3cc75a68d404ca0e1ca2b4831fd2bac4bb/src/util/system.cpp#L323-L326) and [more ignored arguments](https://github.com/ryanofsky/bitcoin/blob/69d44f3cc75a68d404ca0e1ca2b4831fd2bac4bb/src/util/settings.cpp#L67-L72), and config file [reverse precedence](https://github.com/ryanofsky/bitcoin/blob/69d44f3cc75a68d404ca0e1ca2b4831fd2bac4bb/src/util/settings.cpp#L61-L65), [inconsistently applied top-level settings](https://github.com/ryanofsky/bitcoin/blob/69d44f3cc75a68d404ca0e1ca2b4831fd2bac4bb/src/util/settings.cpp#L55-L59), and [zombie values](https://github.com/ryanofsky/bitcoin/blob/69d44f3cc75a68d404ca0e1ca2b4831fd2bac4bb/src/util/settings.cpp#L101-L108).

  The original motivation for this change was to make it easy to add a new persistent setting source without introducing more bugs and inconsistencies. Two commits building on top of this to add a persistent `-wallet` setting are pretty straightforward and show how the new code can be extended:

  * 70675c3e4975203ad6222ba2b00c83b4e4213793 from #15935 – _Add \<datadir>/settings.json persistent settings storage_
  * 04c80c40df9fc6f4734ba238ea7f65607cf88089 from #15937 – _Add loadwallet and createwallet RPC load_on_startup options_

ACKs for top commit:
  ariard:
    ACK 083c954
  jnewbery:
    ACK 083c954b02a4e7d0708349eeaf3bac2b5947fb0e
  jamesob:
    ACK 083c954b02a4e7d0708349eeaf3bac2b5947fb0e

Tree-SHA512: 5d106746a44d64d3963c4ef3f4a2fa668a4bedcc9018d3ea12c86beae2fda48a0b036241665837f68685712366f70f2e1faba84d193fa1f456013503097b7659

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