Multidomain trusted browser cookies

Problem/Motivation

When using the trusted browser setting for a multidomain site, the cookie is not set properly leading to a faulty login on a specific domain with a trusted browser because the global cookie_domain variable is set as the default when submitting the configuration form (admin/config/people/tfa).

Proposed resolution

Proposal 1

Use the global cookie_domain variable to set the cookie and remove the cookie domain setting in the configuration form.

Proposal 2

Make it possible to add multiple cookie domains and add a check when setting the cookie to determine if the domain is allowed to be trusted.

Remaining tasks

I've included 2 patches for both proposals that could use a review.