Birthdate field under discussion also in Arch Linux

Add a required birth date prompt (YYYY-MM-DD) to the user creation flow, stored as a systemd userdb JSON drop-in at /etc/userdb/<user>.user on the target system.

Motivation

Recent age verification laws in California (AB-1043), Colorado (SB26-051), Brazil (Lei 15.211/2025), etc. require platforms to verify user age. Collecting birth date at install time ensures Arch Linux is compliant with these regulations.

This is just a pull request, no changes yet.

The pull-request discussion thread has been locked, just like it happened for the similar thread in Systemd, owing to the amount of negative comments…