make lxc.network.script.down consistent with lxc.network.script.up

- add missing print in lxc_list_nicconfigs() and in write_config() which
  fixes lxc-clone so it will propagate the script.down config value

- add missing "script.down" to lxc_get_item_nic(). change "upscript" to
  "script.up" to match actual key in config

- refactor to use config_string_item()

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Acled-by: Stéphane Graber <stgraber@ubuntu.com>