authelia's file backend has no self-service reset (no SMTP notifier),
so the only way a human account got a new password after the old one
was forgotten was hand-editing users.yml as root. `user add` already
hashes a password into the file; this verb does the same for an
existing user instead of refusing on the name.
Mirrors `user add`'s UX exactly: no password flag, authelia generates
and hashes it (never crosses argv), and it's printed once and never
stored. Refuses on an unknown user before ever invoking authelia. Same
publish path as add/update, so the same atomic write and no-restart
(authelia watches the file) behaviour apply.
Split the digest-replacement into users::reset_password so it's
testable without a command line or a running authelia, same pattern
as apply_update.
69ae23f801