system: prevent carp demotion, closes #1100

Suggested by: @andreas-p
Inspired by:  pfSense

(cherry picked from commit 79a2008b)

src/etc/inc/system.inc

@@ -119,6 +119,8 @@ function activate_sysctls()
         "net.enc.in.ipsec_filter_mask" => "0x0002",
         "net.enc.out.ipsec_bpf_mask" => "0x0001",
         "net.enc.out.ipsec_filter_mask" => "0x0001",
+        'net.inet.carp.senderr_demotion_factor' => '0',
+        'net.pfsync.carp_demotion_factor' => '0',
     );
 
     if (isset($config['sysctl']['item'])) {