yast2-users: OS command injection via LDAP-supplied shadowLastChange/shadowExpire attribute

An OS command injection vulnerability was found in yast2-users. When displaying the "Password Settings" tab of a user, get_password_term() in src/include/users/dialogs.rb read the shadowLastChange and shadowExpire fields with GetString(), which performs no numeric validation, and passed the resulting string to format_days_after_epoch(). That helper interpolated the value into a shell command executed via Ruby backticks without quoting or escaping. Impact: an administrator who manages users against an external/federated LDAP directory via `yast2 users` triggers root command execution the moment they view or edit that particular user's "Password Settings" tab. No "join domain" or trust setup is required, just browsing/editing one user entry. This issue affects yast2-users through 5.0.8.