common: Allow a readlimit for iobuf_esopen.
* common/iobuf.c (file_es_filter_ctx_t): Add fields use_readlimit and readlimit. (file_es_filter): Implement them. (iobuf_esopen): Add new arg readlimit. * g10/decrypt-data.c (decrypt_data): Adjust for change. * g10/import.c (import_keys_es_stream): Ditto.
This comes handy for (length,datablob) style streams.
- Signed-off-by: Werner Koch <wk@gnupg.org>