enhanced set_pkcs12_cred test.
doc update
simplified x509dn
The RDN sequence is now kept in trust list instead of the credentials parameters.

This is however not enabled by default. When adding CAs to trust list
the flag GNUTLS_TL_USE_IN_TLS must be specified to generate the RDN
sequence. This flag is for now only useful internally in gnutls.
doc update
gnutls_session_get_desc() returns a more compact description.