sbbs

If an original references header field was supplied with the message, include
*that* in the header (since it should contain all the IDs of the replied-to
message thread) rather than just the message's reply_id (a single message
ID) when sending via NNTP. This maintains more NNTP post integrity when
sending back out via NNTP.

Not sure why we were storing received "references" in the hdr.references
returned by parse_news_header(), we don't seem to be using it anywere. Maybe
copied from 822header.js or mailproc_util.js? Seems like cruft, so remove it.

Also, don't ever send an "In-Reply-To" heder field via NNTP (it's not valid
/used for NNTP articles, per RFC 5536).