Tag node bash processes and add attach script

Try invoking bash processes with -s mininet:host,
for easy identification of hosts. This enables
easy attachment using the util/m script.
closes #121

util/m

@@ -0,0 +1,30 @@
+#!/bin/bash
+
+# Attach to a Mininet host and run a command
+
+if [ -z $1 ]; then
+  echo "usage: $0 host cmd [args...]"
+  exit 1
+else
+  host=$1
+fi
+
+pid=`pgrep -f mininet:$host`
+if [ "$pid" == "" ]; then
+  echo "Could not find Mininet host $host"
+  exit 2
+fi
+
+if [ -z $2 ]; then
+  cmd='bash'
+else
+  shift
+  cmd=$*
+fi
+
+cgroup=/sys/fs/cgroup/cpu/$host
+if [ -d "$cgroup" ]; then
+  cg="-g $host"
+fi
+
+exec sudo mnexec -a $pid $cg $cmd