Really sort the "Switch user" list

The "Switch user" list is made up of two sub-lists, one with the users that have the permission to switch (in italics) and, if they are less than 10, a second list with 'normal' users (in roman). Right now, each sub-list is sorted by last access date.

I propose to sort the overall list by last access date. You can still distinguish the two categories of users, because they're either italic or roman, so we wouldn't lose any information, but we'd actually gain some information, because we'd really see the ten last active users in their proper order.

Attached is a patch for D7 for discussion. If this is acceptable, I'll provide the ones for D6 and D5, too, of course.