auth ldap/ad: introduce connection 'mode'

auth ldap/ad: introduce connection 'mode'

instead of having only a 'secure' flag which switches between
ldap/ldaps we now have a mode which also contains 'ldap+starttls'

our connection code in PVE::LDAP can handle this already (used in pmg)
so that is no problem

if we want to really remove the 'secure' flag, e.g. in 7.0
we'd either have to rewrite the config or have it as an error
in a pve6to7 script

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>