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The only reason for including the gssapi.h from the dst/gssapi.h header
was to get the typedefs of gss_cred_id_t and gss_ctx_id_t.  Instead of
using those types directly this commit introduces dns_gss_cred_id_t and
dns_gss_ctx_id_t types that are being used in the public API and
privately retyped to their counterparts when we actually call the gss
api.

This also conceals the gssapi headers, so users of the libdns library
doesn't have to add GSSAPI_CFLAGS to the Makefile when including libdns
dst API.