[leafnode-list] inaccurate response to GROUP command

[Brian Sammon]

> leafnode will _NEVER_ decrement the "high watermark" even if some of the
> articles with higher numbers have expired.
> 
> Reasons: newsreaders look at the article number which is monotonically
> increasing, if it corrected the figures to 211 1 3 3, the newsreader
> might either not present the next 3 articles to arrive (they would be
> assigned 4 to 6 again which the newsreader considers read since they
> have been there) or would re-fetch the whole group.

nntpext says that "the high water mark can decrease if an article is removed"

What leafnode _could_ do while staying compliant is:
Internally, maintain a "highest message number ever" pointer and a "highest 
current message number" pointer.  Return the "highest current message number" 
to GROUP and LIST commands, while using the "highest message number ever" to 
chose a message number for new messages.

Interestingly, while RFC977 is vague about the GROUP response, it _is_ quite 
clear that the response to LIST command must reference "the last known article 
currently in that newsgroup".  nntpext replaces this with the vaguer "the 
reported high water mark"

Anyways, I've figured out how to fix the touch_newsgroup script, so this is 
all academic.