file system: enhanced file status info

This patch extends the 'File_system::Status',
'File_system::Directory_entry', and the related 'Vfs' types with
the following additional information:

- Distinction between continuous and transactional files (Node_type)
  (issue #3507)
- Readable, writeable, and executable attributes (Node_rwx),
  replacing the former 'mode' bits
  (issue #3030)

The types 'Node_rwx', 'Node_type' are defined twice,
once for the VFS (vfs/types.h) and once for the 'File_system'
session (file_system_session/file_system_session.h).
Similarly, there is a direct correspondance between
'Vfs::Directory_service::Dirent' and 'File_system::Directory_entry'.

This duplication of types follows the existing pattern of keeping the
VFS and file-system session independent from each other.

repos/os/src/lib/vfs/rom_file_system.h

@@ -107,7 +107,8 @@ class Vfs::Rom_file_system : public Single_file_system
 		Rom_file_system(Vfs::Env &env,
 		                Genode::Xml_node config)
 		:
-			Single_file_system(NODE_TYPE_FILE, name(), config),
+			Single_file_system(Node_type::CONTINUOUS_FILE, name(),
+			                   Node_rwx::ro(), config),
 
 			/* use 'label' attribute if present, fall back to 'name' if not */
 			_label(config.attribute_value("label",
@@ -161,10 +162,12 @@ class Vfs::Rom_file_system : public Single_file_system
 			 * found a file), obtain the size of the most current ROM module
 			 * version.
 			 */
-			if (out.mode == STAT_MODE_FILE) {
+			if (out.type == Node_type::CONTINUOUS_FILE) {
 				_rom.update();
 				out.size = _rom.valid() ? _rom.size() : 0;
-				out.mode |= 0555;
+				out.rwx  = { .readable   = true,
+				             .writeable  = false,
+				             .executable = true };
 			}
 
 			return result;