This commit is a preparatory stage for removing ssh_buffer_get_begin().
Note that removing ssh_buffer_get_begin() doesn't break API
compatibility, as this functions has never been exposed (it only has the
LIBSSH_API prefix).
Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>