selinux: use SELinux status page

Switch from security_getenforce() and netlink notifications to the
SELinux status page.

This usage saves system calls and will also be the default in
libselinux > 3.1 [1].

[1]: https://github.com/SELinuxProject/selinux/commit/05bdc03130d741e53e1fb45a958d0a2c184be503

src/core/selinux-access.c

@@ -198,7 +198,7 @@ int mac_selinux_generic_access_check(
                 return r;
 
         /* delay call until we checked in `access_init()` if SELinux is actually enabled */
-        enforce = security_getenforce() != 0;
+        enforce = mac_selinux_enforcing();
 
         r = sd_bus_query_sender_creds(
                         message,
@@ -278,7 +278,7 @@ int mac_selinux_generic_access_check(
         return enforce ? r : 0;
 }
 
-#else
+#else /* HAVE_SELINUX */
 
 int mac_selinux_generic_access_check(
                 sd_bus_message *message,
@@ -289,4 +289,4 @@ int mac_selinux_generic_access_check(
         return 0;
 }
 
-#endif
+#endif /* HAVE_SELINUX */