add LXC_CGNS_AWARE env variable for mount hooks

doc/lxc.container.conf.sgml.in

@@ -1593,6 +1593,22 @@ mknod errno 0
           </listitem>
         </varlistentry>
       </variablelist>
+      <variablelist>
+        <varlistentry>
+          <term>
+            <option>LXC_CGNS_AWARE</option>
+          </term>
+          <listitem>
+            <para>
+	      If unset, then this version of lxc is not aware of cgroup
+	      namespaces.  If set, it will be set to 1, and lxc is aware
+	      of cgroup namespaces.  Note this does not guarantee that
+	      cgroup namespaces are enabled in the kernel.  This is used
+	      by the lxcfs mount hook.
+            </para>
+          </listitem>
+        </varlistentry>
+      </variablelist>
     </refsect2>
     <refsect2>
     <title>Logging</title>

src/lxc/start.c

@@ -451,6 +451,9 @@ struct lxc_handler *lxc_init(const char *name, struct lxc_conf *conf, const char
 	if (conf->console.log_path && setenv("LXC_CONSOLE_LOGPATH", conf->console.log_path, 1)) {
 		SYSERROR("failed to set environment variable for console log");
 	}
+	if (setenv("LXC_CGNS_AWARE", "1", 1)) {
+		SYSERROR("failed to set LXC_CGNS_AWARE environment variable");
+	}
 	/* End of environment variable setup for hooks */
 
 	if (run_lxc_hooks(name, "pre-start", conf, handler->lxcpath, NULL)) {