Add support for gss_{import,export}_cred() as requested by metze

Works for krb5 and SPNEGO mechanisms. Kerberos credentials are passed as
credential cache names, or if there are memory based credentials, inband in the protocol. This means that the credentials buffers must be keep secret.

As documented by IBM (they have the wrong prototype though)
and GGF (GSS-API Extensions) back in 2001

lib/krb5/version-script.map

@@ -710,6 +710,7 @@ HEIMDAL_KRB5_2.0 {
 		# Shared with GSSAPI krb5
 		_krb5_crc_init_table;		
 		_krb5_crc_update;		
+		_krb5_get_krbtgt;
 
 		# V4 compat glue
 		_krb5_krb_tf_setup;