Add helper APIs to the mechanism glue from the Apple implementation.
gss_mg_export_name() can be used by a mechanism to create an exported name
(this is used by the future SAnon implementation). _gss_mech_import_name()
allows the mechanism to pass in a list of parsing functions for specific name
tiypes. This commit only adds support to the mechanism glue itself.
The GSS mechglue uses singly linked lists for mechanisms and mechanism objects,
to which new entries are inserted at the head. This breaks ordering of
mechanisms specified in OID sets and in /etc/gss/mech, as they will be back to
front. Use a tail queue instead so that new entries are inserted at the end.
