Provide hard-coded dummy implementation of parse_arguments()

This sets argv[0] to "echo" always and returns 1 for argc, so we have something at least that works for demonstration (otherwise, load() will fail because there's no file name)

userprog/parse_arguments.c

@@ -2,5 +2,10 @@
 
 
 int parse_arguments(const char* command_line, char**argv) {
-    #warning "Implement me"
+    /*
+     * XXX: Dummy implementation which sets argv[0] to "echo" so at least there's
+     * a filename to load.
+     */
+    argv[0] = "echo";
+    return 1;
 }