Deduplicate bitcoind and bitcoin-qt init code

Add common InitConfig function to deduplicate bitcoind and bitcoin-qt code
reading config files and creating the datadir.

There are a few minor changes in behavior:

- In bitcoin-qt, when there is a problem reading the configuration file, the
  GUI error text has changed from "Error: Cannot parse configuration file:" to
  "Error reading configuration file:" to be consistent with bitcoind.
- In bitcoind, when there is a problem reading the settings.json file, the
  error text has changed from "Failed loading settings file" to "Settings
  file could not be read" to be consistent with bitcoin-qt.
- In bitcoind, when there is a problem writing the settings.json file, the
  error text has changed from "Failed saving settings file" to "Settings file
  could not be written" to be consistent with bitcoin-qt.
- In bitcoin-qt, if there datadir is not accessible (e.g. no permission to read),
  there is an normal error dialog showing "Error: filesystem error: status:
  Permission denied [.../settings.json]", instead of an uncaught exception

src/common/init.h

@@ -0,0 +1,39 @@
+// Copyright (c) 2023 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_COMMON_INIT_H
+#define BITCOIN_COMMON_INIT_H
+
+#include <util/translation.h>
+
+#include <functional>
+#include <optional>
+#include <string>
+#include <vector>
+
+class ArgsManager;
+
+namespace common {
+enum class ConfigStatus {
+    FAILED,       //!< Failed generically.
+    FAILED_WRITE, //!< Failed to write settings.json
+    ABORTED,      //!< Aborted by user
+};
+
+struct ConfigError {
+    ConfigStatus status;
+    bilingual_str message{};
+    std::vector<std::string> details{};
+};
+
+//! Callback function to let the user decide whether to abort loading if
+//! settings.json file exists and can't be parsed, or to ignore the error and
+//! overwrite the file.
+using SettingsAbortFn = std::function<bool(const bilingual_str& message, const std::vector<std::string>& details)>;
+
+/* Read config files, and create datadir and settings.json if they don't exist. */
+std::optional<ConfigError> InitConfig(ArgsManager& args, SettingsAbortFn settings_abort_fn = nullptr);
+} // namespace common
+
+#endif // BITCOIN_COMMON_INIT_H