userland: move more multithreading from cpp-cheat!

Convert infinite_loop.c into loop.c. Keep all examples fast by default!

userland/cpp/thread_get_id.cpp

@@ -0,0 +1,29 @@
+// https://cirosantilli.com/linux-kernel-module-cheat#cpp-multithreading
+//
+// Spawn some threads and print their ID.
+//
+// On Ubuntu 19.04, they ar large possibly non-consecutive numbers.
+
+#include <iostream>
+#include <mutex>
+#include <thread>
+#include <vector>
+
+std::mutex mutex;
+
+void myfunc(int i) {
+    mutex.lock();
+    std::cout << i << " " << std::this_thread::get_id() << std::endl;
+    mutex.unlock();
+}
+
+int main() {
+    std::cout << "main " << std::this_thread::get_id() << std::endl;
+    std::vector<std::thread> threads;
+    for (unsigned int i = 0; i < 4; ++i) {
+        threads.push_back(std::thread(myfunc, i));
+    }
+    for (auto& thread : threads) {
+        thread.join();
+    }
+}