Do not call ip binary with hardcoded path

On new systems is ip binary in /usr/bin/ instead of /sbin/.

As netplug script explicitly sets $PATH with both /usr/bin/ and /sbin/
directories, do not call ip binary with hardcode path and instead call ip
binary without explicit path.

https://bugs.debian.org/1076624

diff --git a/scripts/netplug b/scripts/netplug
index e1d16d5505a3..87b37a840c67 100755
--- a/scripts/netplug
+++ b/scripts/netplug
@@ -34,7 +34,7 @@ in)
 out)
     if [ -x /sbin/ifdown ]; then
 	# At least on Fedora Core 1, the call to ip addr flush infloops
-	# /sbin/ifdown "$dev" && exec /sbin/ip addr flush "$dev"
+	# /sbin/ifdown "$dev" && ip addr flush "$dev"
 	exec /sbin/ifdown "$dev"
     else
 	echo "Please teach me how to unplug an interface!" 1>&2
@@ -42,7 +42,7 @@ out)
     fi
     ;;
 probe)
-    exec /sbin/ip link set "$dev" up >/dev/null 2>&1
+    exec ip link set "$dev" up >/dev/null 2>&1
     ;;
 *)
     echo "I have been called with a funny action of '%s'!" 1>&2