urlsum.txt: summary of results
   urllog.txt: log of progress
   urlok.html: found   2 urls: were ok
 urlhost.html: ERROR   6 hosts: failed
 urlfail.html: ERROR   7 urls: failed
urlfailX.html: ERROR   7 urls: failed (cross referenced)
urlfailF.html: ERROR   2 files with failed urls
 urlskip.html: warn    6 urls: not checked
 urlwarn.html: warn    1 warning
urlwarnX.html: warn    1 warning (cross referenced)
urlwarnF.html: warn    4 urls with warnings