Drupal login/cookie issues (logging into different domains)

As a note to self, when working on a Drupal website, when it comes to logging in, the domain www.alvinalexander.com is not the same as alvinalexander.com or m.alvinalexander.com, even if they all point to the same Drupal multisite installation and share the same database. There may be some way to configure a Drupal cookie so that logging into one domain will log you into all of them, but right now, that’s the way it works (by default).