journalctl: tighten rules on parsing namespace journal dir suffixes

journalctl: tighten rules on parsing namespace journal dir suffixes

The dot must follow the machine ID immediately, let's check for that.
Also, I think it's generally better to parse the machine ID and then
comparing it, instead of comparing the string representation. That's
more in line how we usually do it, as we parse 128bit IDs generally
case-insensitively.