Basic ^C interrupt implementation.

For testing, I made cat put itself into a new process group.
This should eventually be done by sh between fork() and exec().

Userland/cat.cpp

@@ -5,6 +5,7 @@
 
 int main(int argc, char** argv)
 {
+    setpgid(0, 0);
     if (argc != 2) {
         printf("usage: cat <file>\n");
         return 1;