userland: move some multithreaded examples from cpp-cheat

Using them mostly to evaluate how well the emulators are handling user
mode multithreading.

userland/posix/count_to.c

@@ -0,0 +1,29 @@
+/* Count up to a given number 1 second sleep between each increment.
+ *
+ * https://github.com/cirosantilli/linux-kernel-module-cheat#unistd-h
+ *
+ * We need a separate program for this from count.c because count.c
+ * is also usable as an init process, where we can't control the CLI
+ * arguments very well.
+ */
+
+#define _XOPEN_SOURCE 700
+#include <limits.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+int main(int argc, char **argv) {
+    unsigned long i, max;
+    if (argc > 1) {
+        max = strtoll(argv[1], NULL, 0);
+    } else {
+        max = 1;
+    }
+    i = 0;
+    while (i < max) {
+        printf("%lu\n", i);
+        i++;
+        sleep(1);
+    }
+}