Make dns_glue_t private to qpzone

Make dns_glue_t private to qpzone

The dns_glue_t, dns_gluelist_t and dns_glue_additionaldata_ctx types are
only used in qpzone.c. This commits moves them to the private header
qpzone_p.h.

This is done in preparation of a followup commit that will refactor them
to use types that are private to qpzone.