fs/Path: rename to AllocatedPath

The new class Path only holds a string pointer without being
responsible for allocation/deallocation.  The FileSystem.hxx library
accepts Path arguments instead of AllocatedPath, to avoid forcing
callers to allocate another string object.

src/TextFile.hxx

@@ -36,7 +36,7 @@ class TextFile {
 	GString *const buffer;
 
 public:
-	TextFile(const Path &path_fs);
+	TextFile(Path path_fs);
 
 	TextFile(const TextFile &other) = delete;