Add a shim to znc

 This serves to do two things:

  1) chown /znc to znc:znc, which is really handy if the znc
     user inside the docker container has a new uid, because the
     files are stored on the host filesystem.
  2) to drop privs to the znc user, to reduce the attack surface.
Merge pull request #1 from paultag/master

Add a shim to znc