Make sure exec_type is assigned to all entrypoint types.

Some file types used as domain entrypoints were missing the
exec_type attribute.  Add it and add a neverallow rule to
keep it that way.

Change-Id: I7563f3e03940a27ae40ed4d6bb74181c26148849
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
diff --git a/su.te b/su.te
index ca9fcc2..c1f002f 100644
--- a/su.te
+++ b/su.te
@@ -1,6 +1,6 @@
 type su, domain;
 permissive su;
-type su_exec, file_type;
+type su_exec, exec_type, file_type;
 domain_auto_trans(shell, su_exec, su)
 
 # su is unconfined.