Merge #10267: New -includeconf argument for including external configuration files

25b7ab9 doc: Add release notes for -includeconf (Karl-Johan Alm)
0f0badd test: Test includeconf parameter. (Karl-Johan Alm)
629ff8c -includeconf=<path> support in config handler, for including external configuration files (Karl-Johan Alm)

Pull request description:

  Fixes: #10071.

  Done:
  - adds `-includeconf=<path>`, where `<path>` is relative to `datadir` or to the path of the file being read, if in a file
  - protects against circular includes
  - updates help docs

  ~~~Thoughts:~~~
  - ~~~I am not sure how to test this in a neat manner. Feedback on this would be nice. Will dig/think though.~~~

Tree-SHA512: cb31f1b2f69fbc0890d264948eb2e501ac05cf12f5e06a5942f9c1539eb15ea8dc3cae817f4073aecb2fcc21d0386747f14f89d990772003a76e2a6d25642553

src/bitcoind.cpp

@@ -92,7 +92,7 @@ static bool AppInit(int argc, char* argv[])
         }
         try
         {
-            gArgs.ReadConfigFile(gArgs.GetArg("-conf", BITCOIN_CONF_FILENAME));
+            gArgs.ReadConfigFiles();
         } catch (const std::exception& e) {
             fprintf(stderr,"Error reading configuration file: %s\n", e.what());
             return false;