Many misc changes.

- Faster HTML parser derivede from SGMLparser (Fred Gansevles).

- All manipulations of todo, done, ext, bad are done via methods, so a
derived class can override.  Also moved the 'done' marking to
dopage(), so run() is much simpler.

- Added a method status() which returns a string containing the
summary counts; added a "total" count.

- Drop the guessing of the file type before opening the document -- we
still need to check those links for validity!

- Added a subroutine to close a connection which first slurps up the
remaining data when it's an ftp URL -- apparently closing an ftp
connection without reading till the end makes it hang.

- Added -n option to skip running (only useful with -R).

- The Checker object now has an instance variable which is set to 1
when it is changed.  This is not pickled.