Merge #19131: refactor: Fix unreachable code in init arg checks

eea81146571480b2acd12c8cd7f36b04d056c47f build: Enable unreachable-code-loop-increment (Jonathan Schoeller)
d15db4b1fc988736b08c092d000ca1d1ff686975 refactor: Fix unreachable code in init arg checks (Jonathan Schoeller)

Pull request description:

  Closes: #19017

  In #19015 it's been suggested that we add some new compiler warnings to our build. Some of these, such as `-Wunreachable-code-loop-increment`, generate warnings. We'll likely want to fix these up if we're going to turn these warnings on.

  ```shell
  init.cpp:969:5: warning: loop will run at most once (loop increment never executed) [-Wunreachable-code-loop-increment]
       for (const auto& arg : gArgs.GetUnsuitableSectionOnlyArgs()) {
       ^~~
   1 warning generated.
   ```
   https://github.com/bitcoin/bitcoin/blob/aa8d76806c74a51ec66e5004394fe9ea8ff0fac4/src/init.cpp#L968-L972

  To fix this, collect all errors, and output them in a single error message after the loop completes. This resolves the unreachable code warning, and avoids popup hell that could result from outputting a seperate message for each error or warning one by one.

ACKs for top commit:
  laanwj:
    Code review ACK eea81146571480b2acd12c8cd7f36b04d056c47f
  hebasto:
    re-ACK eea81146571480b2acd12c8cd7f36b04d056c47f, only suggested changes applied since the [previous](https://github.com/bitcoin/bitcoin/pull/19131#pullrequestreview-421772387) review.

Tree-SHA512: 2aa3ceb7fab581b6ba2580900668388d8eba1c3001c8ff9c11c1f4a9a10fbc37f30e590249862676858446e3f4950140a252953ba1643ba3bfd772f8eae20583

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