Getting the overview file can take a long time

[LegalizeAdulthood]

Current version trn can take a long time to get the overview file on my local ISP.

Investigate this with the updated trn and see if there's something we can do about speeding this up.

[LegalizeAdulthood]

Using strace it was observed that when entering a newsgroup, trn does a LISTGROUP with no arguments. This results in the server traversing the newsgroup's entire store to identify the valid numbers of all articles. It's probably starting a counter at the minimum article number and doing a file existence check for every number from the low to high article numbers for the group, resulting in a very lengthy and disk intensive process for a newsgroup with many articles, e.g. comp.os.vms on my server.

At a minimum, trn should be using the .newsrc to identify the range of article numbers that need to be listed. It should probably be using the overview support in NNTP to do something better as an alternative.

[LegalizeAdulthood]

This all takes place in find_existing_articles