gss: use tail queue instead of singly linked list in mechglue

gss: use tail queue instead of singly linked list in mechglue

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.