Log Message:
- upgrade to latest snapshot, changes from -15 to -17:

16. src/art.c,etc: If an article has invalid headers, clear the error
    but give the article an initial score of -1000.  This number may
    be customized using the "invalid_header_score" variable.
17. src/mime.c: In slrn_mime_process_article, decode headers even if
    Content-Type is not understood and metamail is needed.