allow AI models to contain <nasal><load> and <nasal><unload> blocks in

allow AI models to contain <nasal><load> and <nasal><unload> blocks in
their XML wrapper/animation file. They can access their /ai/models node
via cmdarg() function. Example:

  <nasal>
          <load>
                  print("Hi, I'm the Nimitz. My data are under ",
                                  cmdarg().getPath());
          </load>
          <unload>
                  ...
          </unload>
  </nasal>

Note, however, that the <unload> block is only called on exit at the moment,
not when the tile is unloaded.