userland: add some random filesystem and random stuff

Some moved from C++, some moved from SO, some I just made up.

userland/cpp/random.cpp

@@ -0,0 +1,22 @@
+// https://cirosantilli.com/linux-kernel-module-cheat#cpp
+
+#include <iostream>
+#include <random>
+
+int main(int argc, char **argv) {
+    // Potentially expensive /dev/urandom read that waits for entropy.
+    std::random_device dev;
+    std::mt19937 prng(dev());
+    std::uniform_int_distribution<uint64_t> zero_ull_max(0);
+    std::uniform_int_distribution<uint8_t> one_to_six(1, 6);
+
+    unsigned long long loops = 1;
+    if (argc > 1) {
+        loops = std::stoull(argv[1], NULL, 0);
+    }
+
+    for (auto i = 0U; i < loops; ++i) {
+        std::cout << "zero_to_ull_max 0x" << std::hex << zero_ull_max(prng) << std::endl;
+        std::cout << "one_to_six " << std::dec << one_to_six(prng) << std::endl;
+    }
+}