New: feed-max-size and feed-max-size-ips

This is all because LiveJournal has undocumented size limits on incoming syndication feeds, limits that DWiki can easily blow past. Since I actually wanted LiveJournal to be able to get syndication feeds from me, DWiki has grown two new configuration settings.

feed-max-size is an integer kilobytes. It is a rough limit on how large any feed can be; once DWiki generates a feed that is this many kilobytes or larger it stopps adding more entries, regardless of the setting for atomfeed-display-howmany. If unset, there is no size limit.

feed-max-size-ips restricts feed-max-size to the whitespace separated list of IP addresses or tcpwrappers style IP address prefixes (eg '66.150.15.' to get all of 66.150.15.*). Syndication fetches from other addresses will behave as if there was no feed-max-size.

Strictly speaking, feed-max-size limits only the size of the atom::pages or atom::comments output to that size. Whatever else is tacked on to make up a feed (hopefully not very big) will add some extra size.

Moral: undersize feed-max-size a bit. For LiveJournal, the limit is apparently 150 kilobytes (currently), so setting it to '120' or so should provide a comfortable safety margin.

Although I'm not entirely fond of this (to put it one way), the documentation has been updated appropriately, making this feature more or less official.