tfa list: account for admin permissions

instead of restricting listing tfa entries of others to
root@pam, perform the same checks the user-list does and
which also reflect the permissions of the api calls actually
operating on those users, so, `User.Modify` on the user (but
also `Sys.Audit`, since it's only a read-operation, just
like the user index API call)

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>