util: Add GetPathArg default path argument

Let GetPathArg method be used more places for path arguments that have
default values, like "-settings" and BITCOIN_SETTINGS_FILENAME in the
next commit.

Also:

- Fix negated argument handling. Return path{} not path{"0"} when path
  argument is negated.

- Add new tests for default and negated cases

- Move GetPathArg() method declaration next to GetArg() declarations.
  The two methods are close substitutes for each other, so this should
  help keep them consistent and make them more discoverable.

src/test/getarg_tests.cpp

@@ -245,6 +245,24 @@ BOOST_AUTO_TEST_CASE(patharg)
 
     ResetArgs(local_args, "-dir=user/.bitcoin/.//");
     BOOST_CHECK_EQUAL(local_args.GetPathArg("-dir"), relative_path);
+
+    // Check negated and default argument handling. Specifying an empty argument
+    // is the same as not specifying the argument. This is convenient for
+    // scripting so later command line arguments can override earlier command
+    // line arguments or bitcoin.conf values. Currently the -dir= case cannot be
+    // distinguished from -dir case with no assignment, but #16545 would add the
+    // ability to distinguish these in the future (and treat the no-assign case
+    // like an imperative command or an error).
+    ResetArgs(local_args, "");
+    BOOST_CHECK_EQUAL(local_args.GetPathArg("-dir", "default"), fs::path{"default"});
+    ResetArgs(local_args, "-dir=override");
+    BOOST_CHECK_EQUAL(local_args.GetPathArg("-dir", "default"), fs::path{"override"});
+    ResetArgs(local_args, "-dir=");
+    BOOST_CHECK_EQUAL(local_args.GetPathArg("-dir", "default"), fs::path{"default"});
+    ResetArgs(local_args, "-dir");
+    BOOST_CHECK_EQUAL(local_args.GetPathArg("-dir", "default"), fs::path{"default"});
+    ResetArgs(local_args, "-nodir");
+    BOOST_CHECK_EQUAL(local_args.GetPathArg("-dir", "default"), fs::path{""});
 }
 
 BOOST_AUTO_TEST_CASE(doubledash)

src/util/system.cpp

@@ -387,9 +387,12 @@ std::optional<unsigned int> ArgsManager::GetArgFlags(const std::string& name) co
     return std::nullopt;
 }
 
-fs::path ArgsManager::GetPathArg(std::string pathlike_arg) const
+fs::path ArgsManager::GetPathArg(std::string arg, const fs::path& default_value) const
 {
-    auto result = fs::PathFromString(GetArg(pathlike_arg, "")).lexically_normal();
+    if (IsArgNegated(arg)) return fs::path{};
+    std::string path_str = GetArg(arg, "");
+    if (path_str.empty()) return default_value;
+    fs::path result = fs::PathFromString(path_str).lexically_normal();
     // Remove trailing slash, if present.
     return result.has_filename() ? result : result.parent_path();
 }

src/util/system.h

@@ -270,16 +270,6 @@ protected:
      */
     std::optional<const Command> GetCommand() const;
 
-    /**
-     * Get a normalized path from a specified pathlike argument
-     *
-     * It is guaranteed that the returned path has no trailing slashes.
-     *
-     * @param pathlike_arg Pathlike argument to get a path from (e.g., "-datadir", "-blocksdir" or "-walletdir")
-     * @return Normalized path which is get from a specified pathlike argument
-     */
-    fs::path GetPathArg(std::string pathlike_arg) const;
-
     /**
      * Get blocks directory path
      *
@@ -342,6 +332,18 @@ protected:
      */
     std::string GetArg(const std::string& strArg, const std::string& strDefault) const;
 
+    /**
+     * Return path argument or default value
+     *
+     * @param arg Argument to get a path from (e.g., "-datadir", "-blocksdir" or "-walletdir")
+     * @param default_value Optional default value to return instead of the empty path.
+     * @return normalized path if argument is set, with redundant "." and ".."
+     * path components and trailing separators removed (see patharg unit test
+     * for examples or implementation for details). If argument is empty or not
+     * set, default_value is returned unchanged.
+     */
+    fs::path GetPathArg(std::string arg, const fs::path& default_value = {}) const;
+
     /**
      * Return integer argument or default value
      *