To setup the CGI for offrss, I advise to write a wrapper that will run the program from the path you are interested, that of feedurls.txt and files/.
Using a script:
#!/bin/sh cd /path/to/feeds exec /path/to/offrss
Using a C wrapper, which you can compile and later setuid:
#include <unistd.h>
int main()
{
int res;
res = chdir("/path/to/feeds");
if (res == -1)
return -1;
return execl("/path/to/offrss", "/path/to/offrss", 0);
}