on 2nd ajax call, behaviors are called with empty settings

Split out from #934234: AJAX requests during drag-and-drop on the "Manage display" tabs in the Field UI breaks the page, which got a little confused.

In short : on some pages with ajax interactions, the second ajax call breaks all behaviors.
Can be seen with poll.module's "More choices", or the ajax interactions on Field UI's "Manage display", but *not* for Field's "Add another item" button - more on that below.

Let's consider the poll case, which is a bit simpler to follow than field UI's "Manage fields" - steps to reproduce :
- enable poll.module
- go to node/add/poll, fill in some text in the existing two 'Choice' textfields
- Press "More choices" once : OK, one more textfield shows up, tabledrag handlers are present
- Press "More choices" once more : one more textfield shows up, but tabledrag handlers are gone

What happens is :

1 - On initial page load, ajax.js initializes an ajax object associated to the "More choices" button.
It starts without an ajax.settings property (i.e ajax.settings == undefined)

2 - 1st ajax call : ajax.success() is called, performs its job fine, and then does :

  (end of Drupal.ajax.prototype.success :)
  // Remove any response-specific settings so they don't get used on the next
  // call by mistake.
  this.settings = { };

3 - No new ajax object is created, *the same ajax object is still associated to the "More choices" button*.

4 - 2nd ajax call : ajax.success() is called :
Drupal.ajax.prototype.commands.settings() merges incoming settings into Drupal.settings - OK
Drupal.ajax.prototype.commands.insert() calls behaviors on the newly inserted HTML, with the following settings :

var settings = response.settings || ajax.settings || Drupal.settings;

For the 1st ajax call, response.settings and ajax.settings were null or undefined, so this evaluates to Drupal.settings - OK
For this second ajax call, however, ajax.settings is now { } (empty object), and the above evaluates to { } : behaviors are called with empty settings instead of Drupal.settings, and tabledrag behavior is not reattached.

The bug does not affect the "Add another item" button on regular, multivalued Fields because, in this case, a new ajax object is created for the "Add" button after each ajax request, and thus, ajax.settings is always undefined and response.settings || ajax.settings || Drupal.settings always evaluates to the expected Drupal.settings.

That's all I have so far. I didn't search yet why in some cases a new ajax object is created for the triggering element, and in some other cases the same object is reused (which causes the bug). I also have no idea either why the bug appears now - I know for sure that the ajax interactions on "Manage display" worked OK a couple weeks ago - however, I cannot really pinpoint a new behavior introduced in, for instance, #561858: [Tests added] Fix drupal_add_js() and drupal_add_css() to work for AJAX requests too by adding lazy-load to AJAX framework.

The naive patch below fixes the bug for me, but it's very possible that this is not the correct fix at all.
I'm also attaching a patch that adds the debug messages I used to trace the behavior above.