* g10/packet.h (struct parse_packet_ctx_s): New.
(parse_packet_ctx_t): New type.
(init_parse_packet): New macro.
* g10/parse-packet.c (parse_packet, dbg_parse_packet): Change to take
a parse context.  Change all callers to provide a context instead of
directly supplying the input stream.
(search_packet, dbg_search_packet): Ditto.
(copy_all_packets, dbg_copy_all_packets): Init an use a parse context.
(copy_some_packets, dbg_copy_some_packets): Ditto.
(skip_some_packets, dbg_skip_some_packets): Ditto.