functions.sh: limit environment of container processes

Set decent defaults in docker upstart script via make security-config

limit nofile 1000 2000		 # Limit number of open files

limit nproc  1000 2000 		 # Prevent fork bombs

  demo@bro:~$ :(){ :|: & };:
  [1] 21
  demo@bro:~$ -bash: fork: retry: Resource temporarily unavailable
  -bash: fork: retry: No child processes
  $ uptime
   18:04:53 up  2:20,  2 users,  load average: 0.00, 0.04, 0.05

limit fsize  100000000 200000000 # Limit files to 100MB

  demo@bro:~$ yes writeabunchofstuff > bigfile
  File size limit exceeded (core dumped)
  demo@bro:~$ ls -lh bigfile
  -rw-r--r-- 1 demo demo 96M Oct 30 18:06 bigfile

limit cpu    500  500		 # Limit user cpu time