[ATM]: update links to new site (from <mitch@sfgoth.com)

Documentation/networking/atm.txt

 In order to use anything but the most primitive functions of ATM,
 several user-mode programs are required to assist the kernel. These
 programs and related material can be found via the ATM on Linux Web
-page at http://icawww1.epfl.ch/linux-atm/
+page at http://linux-atm.sourceforge.net/
 
 If you encounter problems with ATM, please report them on the ATM
 on Linux mailing list. Subscription information, archives, etc.,
-can be found on http://icawww1.epfl.ch/linux-atm/
+can be found on http://linux-atm.sourceforge.net/