LDAP: skip anonymous bind when clientcert/key is given

It seems that servers associate the client-cert/key with an account, so
doing an explicit anonymous bind then 'logs out' the already verified
user, limiting the search results in some cases

before refactoring to PVE::LDAP, we did not do '$ldap->bind' at all when
there was no bind_dn, but it is not really clear if Net::LDAP does this
automatically when searching (other libraries do this), so leave the
anonymous bind (for compatibility with PMG) but skip it when a client
certificate and key is given.

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