This is a major change that I have been testing and writing locally. I just tagged UNSTABLE4 before this commit so there's a point to go back to if I end up having to roll back.

* Add per-location-field weights and defaults.
* Add location_api_variant() that I will increment when breaking something in the future that might be used from another module. It is now 1.
* Make settings on location_cck work nicely.
* Field settings are now in a table.
* Field settings defaults can now be overridden in the UI.
* Field settings are now weighted. (Draggable in D6!)
* "Force Default" may now be used for all fields.
* Content type settings validation is repaired again.
* Major COW changes (again) -- Should not have any impact on sites, but be careful until the new code is proven.
* Tests were added to ensure COW functionality doesn't break. (Tests were committed earlier)

API Changes:

* If you were using a location_element, the #location_settings is now the *entire* settings array, not $settings['form']['fields']!
* hook_locationapi() 'collection default' and 'default values' ops  have been merged into a single 'defaults' op, which now allows specifying default weight.
* Collection status 3 is now 4. (Relevant for the 'country' field.)

#330318, reported by danielb: Never, ever load anything when an empty id is passed. We have to enforce this on our side because Drupal doesn't do SQL NULLs.

Should fix the issue brought up in #183004 and #326624.

Better fix for #306171: Pass along criteria to location_save() to ensure that the lid conservation code doesn't reuse a lid tied to a single *other instance*.

This fixes a bug when making a change to a location when adding a revision.

This makes using a location_element much easier, and centralizes the code that does required.

Simplify the titles a bit in the directory view, and make the Drupal 5 version match the Drupal 6 version.

This allows us to have multiple user locations, and simplifies the API for associating locations with other things.

The variables used by location got all shuffled around because of this, but I put in compatibility code to keep setting location_maxnum_{nodetype} and location_defaultnum_{nodetype}, because there is a lot of contrib modules checking for them.

B) Made user locations work more like node locations.

C) Allow more than one location to be "required".

D) Reorganize the variables. Now there is a single variable per content type. (location_settings_{nodetype})

E) Make virtual fields hideable, and add the map link as a virtual field.

F) API change: The return value of 'virtual fields' op is now an array of keys and values, where the values are the labels for the hide checkboxes.

Fix map centering when editing locations with LOCATION_LATLON_UNDEFINED coordinates.
See [#317159]

* Fix $op 'delete' on hook_locationapi().
* API CHANGE: Use location_save_locations() with an empty array instead of deleting rows from {location_instance}. This lets us have "real" deletion instead of relying on garbage collection.

Before:
<code>
db_query("DELETE FROM {location_instance} WHERE genid = '%s'", 'mykey:'. $obj->id);
</code>

After:
<code>
$locations = array();
location_save_locations($locations, array('genid' => 'mykey:'. $obj->id));
</code>

* Remove garbage collection routine and UI from settings page. References are checked automatically now.

Although I can't fix the update warnings proper, I can inform the user why they happen, reduce them, and repair the stuff reported in #317823.

Tested on MySQL and PostgreSQL.
Please be careful until this code is fully proven!