introduce a new K_AUTH service to allow offloading the credentials to a

introduce a new K_AUTH service to allow offloading the credentials to a
table for non-crypt(3) authentication. tables configured with auth that
support K_AUTH are asked to check if a user and passwd are valid rather
than asked to provide the password for a user so smtpd does crypt(3) on
its side. helps with cases like ldap or custom auth.

ok op@