* src/context.h (_gcry_ctx_alloc): Add NEXT argurment.
* src/context.c (struct gcry_context): Add NEXT field.
(_gcry_ctx_alloc): Support the NEXT field.
(_gcry_ctx_get_pointer): Allow access to the NEXT field.
(_gcry_ctx_release): Loop following NEXT.
* cipher/pubkey.c (struct pk_single_data): Remove CTX_NEXT.
(release_single_data): Remove.
(_gcry_pk_single_data_push): Call _gcry_ctx_alloc with NEXT.
(_gcry_pk_get_single_data): Use _gcry_ctx_get_pointer to
access NEXT.