interfaces: defang netgraph attach/detach for #1188

We don't particularly need this code and the utility as netgraph
is always attached on load as long as we don't detach and the
attach per interface is a custom feature that requires a kernel
patch we don't want to keep around for 17.1.

Only thing we need to measure is the performance impact of not
detaching netgraph.  This only pertains to ethernet devices
anyway.

We do, however, keep interface_netgraph_needed() around for
reference as it's harmless by default.

Makefile

@@ -92,7 +92,6 @@ CORE_DEPENDS?=		apinger \
 			lighttpd \
 			miniupnpd \
 			mpd5 \
-			ngattach \
 			ntp \
 			openssh-portable \
 			openvpn \

src/etc/inc/interfaces.lib.inc

@@ -220,12 +220,12 @@ function legacy_interface_details($intf)
 
 function legacy_netgraph_attach($ifs)
 {
-    mwexecf('/usr/local/sbin/ngattach %s', array($ifs));
+    /* XXX compat stub for plugins to be removed in 17.1 */
 }
 
 function legacy_netgraph_detach($ifs)
 {
-    mwexecf('/usr/sbin/ngctl msg %s: detach', array($ifs), true);
+    /* XXX compat stub for plugins to be removed in 17.1 */
 }
 
 function legacy_netgraph_rename($tmpifs, $ifs)

src/etc/rc.d/netflow

@@ -81,8 +81,6 @@ setup_interface()
         echo "setup $interface"
     fi;
 
-    # make sure netgraph is attached
-    /usr/local/sbin/ngattach $interface
     # remove earlier setup (if any)
     /usr/sbin/ngctl shutdown netflow_$interface: >/dev/null 2>&1
     # configure netflow for this interface, sending all to localhost