Replace deprecated libtasn1 macros

New macros were introduced in libatsn1 3.0 on 2012-10-28:

• ASN1_TYPE → asn1_node
• ASN1_DATA_NODE → asn1_data_node_st
• ASN1_TYPE_EMPTY → NULL

We have to go back to RHEL 6 and Fedora 16 for such an early version of libtasn1.

Because we have stopped testing and supporting Cent0S 6, we can get rid of these deprecated types.

Fixes #539 (closed).

See also ocserv@30ae9908.