interfaces: better wait for dhclient exit

PR: https://forum.opnsense.org/index.php?topic=4682.0

(cherry picked from commit 844d6b5)
(cherry picked from commit b8c9eb4)
(cherry picked from commit 2d03d19)

src/etc/inc/interfaces.inc

@@ -2453,15 +2453,7 @@ function kill_dhclient_process($interface)
         return;
     }
 
-    $i = 0;
-    while ((($pid = find_dhclient_process($interface)) != 0) && ($i < 3)) {
-        /* 3rd time make it die for sure */
-        $sig = ($i == 2 ? 'KILL' : 'TERM');
-        exec(sprintf('/bin/kill -%s %s', $sig, $pid));
-        sleep(1);
-        $i++;
-    }
-    unset($i);
+    killbypid(find_dhclient_process($interface), 'TERM', true);
 }
 
 function interface_vlan_mtu_configured($realhwif, $mtu)

src/etc/inc/util.inc

@@ -1,7 +1,7 @@
 <?php
 
 /*
-    Copyright (C) 2015-2016 Franco Fichtner <franco@opnsense.org>
+    Copyright (C) 2015-2017 Franco Fichtner <franco@opnsense.org>
     Copyright (C) 2004-2007 Scott Ullrich <sullrich@gmail.com>
     Copyright (C) 2003-2004 Manuel Kasper <mk@neon1.net>.
     All rights reserved.
@@ -41,12 +41,14 @@ function killbyname($procname, $sig = 'TERM')
 
 function killbypid($pidfile, $sig = 'TERM', $waitforit = false)
 {
-    if (!isvalidpid($pidfile)) {
+    if (is_numeric($pidfile) && $pidfile > 1) {
+        mwexecf('/bin/kill -%s %s', array($sig, $pidfile));
+    } elseif (isvalidpid($pidfile)) {
+        mwexecf('/bin/pkill -%s -F %s', array($sig, $pidfile));
+    } else {
         return;
     }
 
-    mwexecf('/bin/pkill -%s -F %s', array($sig, $pidfile));
-
     if (!$waitforit) {
         return;
     }
@@ -58,11 +60,13 @@ function killbypid($pidfile, $sig = 'TERM', $waitforit = false)
 
 function isvalidpid($pidfile)
 {
-    if (!file_exists($pidfile)) {
-        return false;
+    if (is_numeric($pidfile) && $pidfile > 1) {
+        return mwexecf('/bin/kill -0 %s', $pidfile, true) == 0;
+    } elseif (file_exists($pidfile)) {
+        return mwexecf('/bin/pgrep -nF %s', $pidfile, true) == 0;
     }
 
-    return mwexecf('/bin/pgrep -nF %s', $pidfile, true) == 0;
+    return false;
 }
 
 function is_process_running($process)