(legacy, vpn) some setups seem to have empty mode tags

(cherry picked from commit 74d908fc)
(cherry picked from commit 9bf6c144)

src/etc/inc/plugins.inc.d/vpn.inc

@@ -112,7 +112,7 @@ function vpn_pptpd_configure()
 
     killbypid('/var/run/pptp-vpn.pid', 'TERM', true);
 
-    if (!isset($pptpdcfg['mode']) || $pptpdcfg['mode'] == 'off') {
+    if (!isset($pptpdcfg['mode']) || ($pptpdcfg['mode'] != 'server' && $pptpdcfg['mode'] != 'redir')) {
         return 0;
     }
 
@@ -364,7 +364,7 @@ function vpn_pppoe_configure(&$pppoecfg)
 
     killbypid("/var/run/pppoe{$pppoecfg['pppoeid']}-vpn.pid", 'TERM', true);
 
-    if (!isset($pppoecfg['mode']) || $pppoecfg['mode'] == 'off') {
+    if (!isset($pppoecfg['mode']) || $pppoecfg['mode'] != 'server') {
         return 0;
     }
 
@@ -577,7 +577,7 @@ function vpn_l2tp_configure()
         return 0;
     }
 
-    if (!isset($l2tpcfg['mode']) || $l2tpcfg['mode'] == 'off') {
+    if (!isset($l2tpcfg['mode']) || $l2tpcfg['mode'] != 'server') {
         return 0;
     }