Record news more compactly

Record news more compactly

Member nws_uid is unused since the commit before previous.  Remove it.

Member nws_seqno is of marginal value, because we write news only
through ncache[], and thus aren't prone to the errors sequence numbers
can catch.  Remove it.

Make timestamp selector virtual, computing nws_when + nws_duration,
and remove member nws_timestamp.  Impact:

* In ncache(), the removed timestamp equals nws_when + nws_duration,
  both for new news and updated news.  No change.

* delete_old_news() becomes invisible.  Before, its move of unexpired
  news to the beginning of the news file touched all the timestamps.
  That was unwanted, because the move does not change news, only their
  storage.  Improvement.

* empdump no longer flags the imported news changed via the timestamp.
  This is somewhat unfortunate.  Document as bug.

With these members removed, struct nwsstr no longer matches struct
emptypedstr, so clear news table flag EFF_TYPED and remove union
empobj_storage member news.  This loses the automatic maintenance of
member ef_type via struct emptypedstr.  Remove ef_type as well.

This shrinks struct nwsstr from 20 to 12 bytes on common 32 bit
machines, and from 32 to 16 bytes on common 64 bit machines.  Since
the server doesn't map the whole news file (EFF_MAP is off), this
reduces I/O, while the table's memory use remains the same.

Historical note: struct nwsstr is now pretty much what it was back in
BSD Empire 1.1.  Members ef_type and nws_uid go back to Empire 3 (for
C_SYNC?).  v4.3.12 added member nws_timestamp, which doubled the size
on common 64 bit machines.  v4.3.15 added nws_seqno.