feat(userspace): add demo ring-3 programs

user/counter.c

@@ -0,0 +1,18 @@
+#include "ulib.h"
+
+int main(void)
+{
+    long pid = userGetpid();
+
+    for (int step = 0; step < 5; step++) {
+        print("counter: pid=");
+        printDec(pid);
+        print(" step=");
+        printDec(step);
+        print("\n");
+        userSleep(30);
+    }
+
+    print("counter: finished counting\n");
+    return 0;
+}