interfaces: wait for dhcp6c to exit et al for #1202

(cherry picked from commit c8524059)

src/etc/inc/interfaces.inc

@@ -1282,11 +1282,7 @@ function interface_bring_down($interface = "wan", $destroy = false, $ifacecfg =
     switch ($ipaddrv6) {
         case "slaac":
         case "dhcp6":
-            $pidv6 = find_dhcp6c_process($realif);
-            if ($pidv6) {
-                exec('/bin/kill ' . $pidv6);
-            }
-            sleep(3);
+            killbypid("/var/run/dhcp6c_{$realif}.pid", 'TERM', true);
             @unlink("/var/etc/dhcp6c_{$interface}.conf");
             if (does_interface_exist($realifv6)) {
                 $ip6 = find_interface_ipv6($realifv6);
@@ -2581,16 +2577,6 @@ function kill_dhclient_process($interface)
     unset($i);
 }
 
-function find_dhcp6c_process($interface)
-{
-    if ($interface && isvalidpid("/var/run/dhcp6c_{$interface}.pid")) {
-        $pid = trim(file_get_contents("/var/run/dhcp6c_{$interface}.pid"), " \n");
-        return intval($pid);
-    }
-
-    return false;
-}
-
 function interface_vlan_mtu_configured($realhwif, $mtu)
 {
     global $config;
@@ -2971,10 +2957,7 @@ function interface_track6_configure($interface = 'lan', $wancfg, $linkupevent =
                  * XXX: Probably DHCPv6 client should handle this autmagically itself?
                  */
                 $parentrealif = get_real_interface($wancfg['track6-interface']);
-                $pidv6 = find_dhcp6c_process($parentrealif);
-                if ($pidv6) {
-                    exec('/bin/kill -HUP ' . $pidv6);
-                }
+                killbypid("/var/run/dhcp6c_{$parentrealif}.pid", 'HUP');
             }
             break;
     }

src/www/interfaces.php

@@ -1250,10 +1250,7 @@ if ($_SERVER['REQUEST_METHOD'] === 'GET') {
 
             if (!empty($old_config['ipaddrv6']) && $old_config['ipaddrv6'] == 'dhcp6' && $new_config['ipaddrv6'] != 'dhcp6') {
                 // change from dhcp to something else, kill dhcp6c
-                $pid = find_dhcp6c_process($old_config['if']);
-                if ($pid) {
-                    exec('/bin/kill ' . $pid);
-                }
+                killbypid("/var/run/dhcp6c_{$old_config['if']}.pid");
             }
 
             // save to config