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/null_file_system.h

@@ -24,7 +24,8 @@ struct Vfs::Null_file_system : Single_file_system
 {
 	Null_file_system(Vfs::Env&, Genode::Xml_node config)
 	:
-		Single_file_system(NODE_TYPE_CHAR_DEVICE, name(), config)
+		Single_file_system(Node_type::CONTINUOUS_FILE, name(),
+		                   Node_rwx::rw(), config)
 	{ }
 
 	static char const *name()   { return "null"; }