userland: add some random filesystem and random stuff

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

README.adoc

@@ -13386,6 +13386,7 @@ Good sanity check for user mode: <<qemu-user-mode-getting-started>>
 *** link:userland/c/stderr.c[]
 *** File IO
 **** link:userland/c/file_write_read.c[]
+**** link:userland/linux/open_o_tmpfile.c[]: https://stackoverflow.com/questions/4508998/what-is-an-anonymous-inode-in-linux/44388030#44388030
 * Fun
 ** link:userland/c/loop.c[]
 
@@ -13542,6 +13543,9 @@ Programs under link:userland/cpp/[] are examples of https://en.wikipedia.org/wik
 ** link:userland/cpp/if_constexpr.cpp[]: C++17 `if constexpr`
 * fstream
 ** link:userland/cpp/file_write_read.cpp[]
+** link:userland/cpp/temporary_directory.cpp[]: https://stackoverflow.com/questions/3379956/how-to-create-a-temporary-directory-in-c/58454949#58454949
+* random
+** link:userland/cpp/random.cpp[]
 
 [[cpp-multithreading]]
 ==== C++ multithreading