Use gpgrt_stream_t for the API of gpgrt_nvc_*.

* src/gpg-error.h.in (gpgrt_nvc_parse): Use gpgrt_stream_t.
(gpgrt_nvc_write): Likewise.

For applications with no GPGRT_ENABLE_ES_MACROS, it's good not to use
estream_t here.

• Fixes-commit: 7ec1f27b60eec95297c5267ab5ebbdb73b263c3f
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>