name change from "hook modules" to "plugins"

name change from "hook modules" to "plugins"

- "hook" is now used only for hook points and hook actions
- the "hook" statement in named.conf is now "plugin"
- ns_module and ns_modlist are now ns_plugin and ns_plugins
- ns_module_load is renamed ns_plugin_register
- the mandatory functions in plugin modules (hook_register,
  hook_check, hook_version, hook_destroy) have been renamed