Special thanks to N. James Bridge for, like several others, pointing
out the problem in the original version. Even more so, for having condensed
the original version to the one included in this last version.
As far as I'm concerned, anyone who wants to use this routine in AIM clients
may use it.
Note: Problems may occur due to values being declared "long", vice "u_long",
if they do occur, try changing it (It works here as it is)
checksum.c - Checksum routine in C