core: Un-deprecate gpgme_data_rewind.

* src/gpgme.h.in (gpgme_data_rewind): Un-deprecate.
* src/data-compat.c (gpgme_data_rewind): Move to ...
* src/data.c (gpgme_data_rewind): here.

That function is very convenient because it is required a lot with
memory streams. It also documents the intention of the caller better
than gpgme_data_seek with its addition parameters and the need to map
system errors. Thus it does not make sense to make it a first class
citizen again.

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