fs/Traits.hxx: implement BuildFS() method

src/fs/Traits.hxx

@@ -86,6 +86,16 @@ struct PathTraits {
 		return strlen(p);
 	}
 
+	/**
+	 * Constructs the path from the given components.
+	 * If either of the components is empty string,
+	 * remaining component is returned unchanged.
+	 * If both components are empty strings, empty string is returned.
+	 */
+	gcc_pure gcc_nonnull_all
+	static string BuildFS(const_pointer a, size_t a_size,
+			      const_pointer b, size_t b_size);
+
 	/**
 	 * Determine the "base" file name of the given UTF-8 path.
 	 * The return value points inside the given string.