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/rtc_file_system.h

@@ -103,7 +103,8 @@ class Vfs::Rtc_file_system : public Single_file_system
 
 		Rtc_file_system(Vfs::Env &env, Genode::Xml_node config)
 		:
-			Single_file_system(NODE_TYPE_CHAR_DEVICE, name(), config),
+			Single_file_system(Node_type::TRANSACTIONAL_FILE, name(),
+			                   Node_rwx::ro(), config),
 			_rtc(env.env()),
 			_set_signal_handler(env.env().ep(), *this,
 			                    &Rtc_file_system::_handle_set_signal)
@@ -136,10 +137,7 @@ class Vfs::Rtc_file_system : public Single_file_system
 
 		Stat_result stat(char const *path, Stat &out) override
 		{
-			Stat_result result = Single_file_system::stat(path, out);
-			out.mode |= 0444;
-
-			return result;
+			return Single_file_system::stat(path, out);
 		}
 
 		Watch_result watch(char const        *path,