xserver_t needs to ender dirs labeled xdm_var_run_t

The LightDM application stores its xauth file in a subdirectory
(/var/run/lightdm/root) which is labeled as xdm_var_run_t. As a result,
X11 (xserver_t) needs search rights to this location.

With this setup, X is run as follows:
  /usr/bin/X :0 -auth /var/run/lightdm/root/:0

Changes since v1:
- Use read_files_pattern instead of separate allow rules

Signed-off-by: Jason Zaman <jason@perfinion.com>
Signed-off-by: Sven Vermeulen <sven.vermeulen@siphos.be>

policy/modules/services/xserver.te

@@ -820,7 +820,7 @@ allow xserver_t xdm_t:shm rw_shm_perms;
 allow xserver_t xdm_var_lib_t:file { getattr read };
 dontaudit xserver_t xdm_var_lib_t:dir search;
 
-allow xserver_t xdm_var_run_t:file read_file_perms;
+read_files_pattern(xserver_t, xdm_var_run_t, xdm_var_run_t)
 
 # Label pid and temporary files with derived types.
 manage_files_pattern(xserver_t, xdm_tmp_t, xdm_tmp_t)