Enable setting --script-security in client mode.

  This is useful in client mode as well, since it allows one to use --route-noexec and --up <cmd> or --route-up <cmd> to create routes manually instead of relying on whatever routes vpn server pushes down to the client.
  mode=server dependency in luci was introduced together with script_security by mmunz back in 2011.with no explanation in the commit why mode=server was there.