init: Error if ignored bitcoin.conf file is found

Show an error on startup if a bitcoin datadir that is being used contains a
`bitcoin.conf` file that is ignored. There are two cases where this could
happen:

- One case reported in
  https://github.com/bitcoin/bitcoin/issues/27246#issuecomment-1470006043
  happens when a bitcoin.conf file in the default datadir (e.g.
  $HOME/.bitcoin/bitcoin.conf) has a "datadir=/path" line that sets different
  datadir containing a second bitcoin.conf file. Currently the second
  bitcoin.conf file is ignored with no warning.

- Another way this could happen is if a -conf= command line argument points
  to a configuration file with a "datadir=/path" line and that specified path
  contains a bitcoin.conf file, which is currently ignored.

This change only adds an error message and doesn't change anything about way
settings are applied. It also doesn't trigger errors if there are redundant
-datadir or -conf settings pointing at the same configuration file, only if
they are pointing at different files and one file is being ignored.

test/functional/test_framework/util.py

@@ -12,14 +12,16 @@ import inspect
 import json
 import logging
 import os
+import pathlib
 import random
 import re
+import sys
 import time
 import unittest
 
 from . import coverage
 from .authproxy import AuthServiceProxy, JSONRPCException
-from typing import Callable, Optional
+from typing import Callable, Optional, Tuple
 
 logger = logging.getLogger("TestFramework.utils")
 
@@ -420,6 +422,22 @@ def get_datadir_path(dirname, n):
     return os.path.join(dirname, "node" + str(n))
 
 
+def get_temp_default_datadir(temp_dir: pathlib.Path) -> Tuple[dict, pathlib.Path]:
+    """Return os-specific environment variables that can be set to make the
+    GetDefaultDataDir() function return a datadir path under the provided
+    temp_dir, as well as the complete path it would return."""
+    if sys.platform == "win32":
+        env = dict(APPDATA=str(temp_dir))
+        datadir = temp_dir / "Bitcoin"
+    else:
+        env = dict(HOME=str(temp_dir))
+        if sys.platform == "darwin":
+            datadir = temp_dir / "Library/Application Support/Bitcoin"
+        else:
+            datadir = temp_dir / ".bitcoin"
+    return env, datadir
+
+
 def append_config(datadir, options):
     with open(os.path.join(datadir, "bitcoin.conf"), 'a', encoding='utf8') as f:
         for option in options: