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.cpp

@@ -0,0 +1,74 @@
+// 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.
+
+#include <common/init.h>
+#include <chainparams.h>
+#include <fs.h>
+#include <tinyformat.h>
+#include <util/system.h>
+#include <util/translation.h>
+
+#include <algorithm>
+#include <exception>
+#include <optional>
+
+namespace common {
+std::optional<ConfigError> InitConfig(ArgsManager& args, SettingsAbortFn settings_abort_fn)
+{
+    try {
+        if (!CheckDataDirOption(args)) {
+            return ConfigError{ConfigStatus::FAILED, strprintf(_("Specified data directory \"%s\" does not exist."), args.GetArg("-datadir", ""))};
+        }
+        std::string error;
+        if (!args.ReadConfigFiles(error, true)) {
+            return ConfigError{ConfigStatus::FAILED, strprintf(_("Error reading configuration file: %s"), error)};
+        }
+
+        // Check for chain settings (Params() calls are only valid after this clause)
+        SelectParams(args.GetChainName());
+
+        // Create datadir if it does not exist.
+        const auto base_path{args.GetDataDirBase()};
+        if (!fs::exists(base_path)) {
+            // When creating a *new* datadir, also create a "wallets" subdirectory,
+            // whether or not the wallet is enabled now, so if the wallet is enabled
+            // in the future, it will use the "wallets" subdirectory for creating
+            // and listing wallets, rather than the top-level directory where
+            // wallets could be mixed up with other files. For backwards
+            // compatibility, wallet code will use the "wallets" subdirectory only
+            // if it already exists, but never create it itself. There is discussion
+            // in https://github.com/bitcoin/bitcoin/issues/16220 about ways to
+            // change wallet code so it would no longer be necessary to create
+            // "wallets" subdirectories here.
+            fs::create_directories(base_path / "wallets");
+        }
+        const auto net_path{args.GetDataDirNet()};
+        if (!fs::exists(net_path)) {
+            fs::create_directories(net_path / "wallets");
+        }
+
+        // Create settings.json if -nosettings was not specified.
+        if (args.GetSettingsPath()) {
+            std::vector<std::string> details;
+            if (!args.ReadSettingsFile(&details)) {
+                const bilingual_str& message = _("Settings file could not be read");
+                if (!settings_abort_fn) {
+                    return ConfigError{ConfigStatus::FAILED, message, details};
+                } else if (settings_abort_fn(message, details)) {
+                    return ConfigError{ConfigStatus::ABORTED, message, details};
+                } else {
+                    details.clear(); // User chose to ignore the error and proceed.
+                }
+            }
+            if (!args.WriteSettingsFile(&details)) {
+                const bilingual_str& message = _("Settings file could not be written");
+                return ConfigError{ConfigStatus::FAILED_WRITE, message, details};
+            }
+        }
+    } catch (const std::exception& e) {
+        return ConfigError{ConfigStatus::FAILED, Untranslated(e.what())};
+    }
+    return {};
+}
+} // namespace common