cinnamon-settings-users: Prevent against allowing upper-case usernames,
this fails silently in Ubuntu/Mint, where it is disallowed by Ubuntu's
'adduser' program.  Note that 'useradd' works fine, but AccountsService
doesn't use this in Ubuntu or Mint.

Also fixes new group names 'seemingly' allowing upper-case names, then
silently .lower()ing them.  Now, show a warning so the user knows.

Fixes #3461