auth: tfa: read tfa.cfg also if the user.cfg entry has no "x" marker

auth: tfa: read tfa.cfg also if the user.cfg entry has no "x" marker

Previously, `user_get_tfa` read the `keys` user attribute from
user.cfg to determine whether a user has second factors configured.
`keys` could contain TOTP secrets or Yubico key IDs (for realms that
require TFA), or the marker "x" to signify that second factors are
defined in tfa.cfg, in which case `user_get_tfa` would additionally
read tfa.cfg.

However, syncing an LDAP realm with `remove-vanished=properties`
erases the `keys` attribute, and thus the "x" marker (unless custom
`sync_attributes` with a mapping for `keys` are defined). This would
allow TFA-enabled users to log in without a second factor after a
realm sync. This issue was first reported in the forum [1].

To fix this issue, `user_get_tfa` now reads tfa.cfg unconditionally,
and thus independently of the value of `keys`. In other words, the "x"
marker is now irrelevant for authentication. The reasoning for this
change is that most current setups define second factors in tfa.cfg
anyway.

Special care is needed to avoid breaking realms that require TFA: In
that case, `user_get_tfa` must fail authentication if neither user.cfg
nor tfa.cfg define any second factors.

This patch changes the behavior of a hypothetical (and not officially
supported) LDAP realm setup in which `sync_attributes: keys=attr` and
`remove-vanished=properties` is used to maintain `keys` in the LDAP
directory. In such a setup, an admin could enable/disable TFA for a
user who has an enabled second factor in tfa.cfg by editing their LDAP
entry and switching between "x" and "". With this patch, TFA is always
enabled for that user.

This patch makes the "x" marker irrelevant for authentication, but PVE
still *writes* it if the user has second factors configured in
tfa.cfg. This behavior is kept for now to avoid issues in cluster
upgrade scenarios, where some nodes that still rely on the "x" marker
could allow logins without a second factor.

[1] https://forum.proxmox.com/threads/130440/

Suggested-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Signed-off-by: Friedrich Weber <f.weber@proxmox.com>