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/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
      *