urlsum.txt: summary of results
   urllog.txt: log of progress
   urlok.html: found   5 urls: were ok
 urlhost.html: ERROR   1 host: failed
 urlfail.html: ERROR   6 urls: failed
urlfailX.html: ERROR   6 urls: failed (cross referenced)
urlfailF.html: ERROR  13 files with failed urls
 urlskip.html: warn    2 urls: not checked