user-accounts: Fix username validation
man useradd says:
Usernames must start with a lower case letter or an underscore, followed by lower case letters, digits, underscores, or dashes. They can end with a dollar sign. In regular expression terms: [a-z_][a-z0-9_-]*[$]?
The current implementation uses different validation rules, so you can enter a username that will never be created. This patch makes it follow
useradd except for starting
_ or trailing
po files may need to be updated (I don't know how to do that).
Discovered and copied from gnome-initial-setup!30.