core: New interface gpgme_data_new_from_estream.

* src/gpgme.h.in (gpgme_data_new_from_estream): New.
* src/data-estream.c: New.
* src/data.h (gpgme_data): New union member e_stream.

The estream functions (gpgrt_fopen et al.) are any waypart of the
required libgpg-error library and thus it makes sense to provide this
convenience interface.

• Signed-off-by: Werner Koch <wk@gnupg.org>