Allow batch assignment of users to domains

Question #1 is this:

When enabling the module, should we batch assign all users to the default domain?

There is a behavior inconsistency, in that users registered _after_ installation are auto-assigned to a domain, so should we not do so for old accounts on module enable?

[Note that we did not do this in D5 because the data was serialized onto the $user object, which would be difficult to do in hook_enable().]

Here is a patch to auto-add users to the primary domain. Note that we deliberately exclude user 0, to avoid accidental security issues if the anon role is given the 'edit domain nodes' permission.

Patch, part two.

I will clean up the first one and then post a merged patch.

Here is the final patch.

Committed to HEAD.