gsskrb5: Add simple name attributes support

This adds Kerberos mechanism support for:

 - composite principal name export/import
 - getting rudimentary name attributes from GSS names using
   gss_get_name_attribute():
    - all (raw) authorization data from the Ticket
    - all (raw) authorization data from the Authenticator
    - transit path
    - realm
    - component count
    - each component
 - gss_inquire_name()
 - gss_display_name_ext() (just for the hostbased service name type
                           though)

The test exercises almost all of the functionality, except for:

 - getting the PAC
 - getting authz-data from the Authenticator
 - getting the transit path

TBD (much) later:

 - amend test_context to do minimal name attribute checks as well
 - gss_set_name_attribute() (to request authz-data)
 - gss_delete_name_attribute()
 - getting specific authorization data elements via URN fragments (as
   opposed to all of them)
 - parsing the PAC, extracting SIDs (each one as a separate value)
 - some configurable local policy (?)
 - plugin interface for additional local policy

lib/gssapi/version-script.map

@@ -4,6 +4,7 @@ HEIMDAL_GSS_2.0 {
 	global:
 #		__gss_c_nt_anonymous;
 		__gss_c_nt_anonymous_oid_desc;
+		__gss_c_nt_composite_export_oid_desc;
 		__gss_c_nt_export_name_oid_desc;
 		__gss_c_nt_hostbased_service_oid_desc;
 		__gss_c_nt_hostbased_service_x_oid_desc;