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/Makefile.am

@@ -59,6 +59,7 @@ krb5src = \
 	krb5/inquire_mechs_for_name.c \
 	krb5/inquire_names_for_mech.c \
 	krb5/inquire_sec_context_by_oid.c \
+	krb5/name_attrs.c \
 	krb5/pname_to_uid.c \
 	krb5/process_context_token.c \
 	krb5/prf.c \
@@ -381,6 +382,8 @@ LDADD = libgssapi.la \
 	$(top_builddir)/lib/krb5/libkrb5.la \
 	$(LIB_roken)
 
+test_names_LDADD = $(LDADD) $(top_builddir)/lib/asn1/libasn1.la
+
 # gss
 
 dist_gsstool_SOURCES = gsstool.c