replace ping6 with ping

reason: after the upgrade of iputils from 20151218 to 20161105
functionality of ping6 and tracepath6 was merged into ping and tracepath.

Ping is now mostly a drop-in replacment for ping6, except that selecting a
specific interface is done by encoding it into the address (ex.: fe80::1%eth0)
rather then specifing it with the `-I` flag.

pkgs/servers/monitoring/nagios/plugins/official-2.x.nix

@@ -16,8 +16,8 @@ stdenv.mkDerivation rec {
   # configured on the build machine).
   preConfigure= "
     configureFlagsArray=(
-      --with-ping-command='/run/wrappers/bin/ping -n -U -w %d -c %d %s'
-      --with-ping6-command='/run/wrappers/bin/ping6 -n -U -w %d -c %d %s'
+      --with-ping-command='/run/wrappers/bin/ping -4 -n -U -w %d -c %d %s'
+      --with-ping6-command='/run/wrappers/bin/ping -6 -n -U -w %d -c %d %s'
     )
   ";