Update the Dovecot authentication template to use `%n` (user part only)
instead of `%u` (full username) in both the `passdb` username format and
the `userdb` home directory path.
This ensures that user lookups and home directory generation rely specifically
on the username portion, excluding the domain part.
This introduces functionality to manage local Dovecot users utilizing a
static `vmail` system user and a flat password file.
Key changes:
- Added `dovecot_users` configuration list to defaults.
- Implemented creation of `vmail` user and group (uid/gid 5000).
- Added logic to generate a random security token using `pwgen` to prefix
user passwords.
- Created `auth-dovecot-users.conf.ext` and `dovecot-users.j2` templates
to handle `passwd-file` authentication.
- Updated `10-auth.conf` to include the new local users configuration.
- Updated README with usage instructions and token details.
