userland: move more multithreading from cpp-cheat!

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

README.adoc

@@ -12845,7 +12845,7 @@ Programs under link:userland/c/[] are examples of https://en.wikipedia.org/wiki/
 *** File IO
 **** link:userland/c/file_write_read.c[]
 * Fun
-** link:userland/c/infinite_loop.c[]
+** link:userland/c/loop.c[]
 
 ==== malloc
 
@@ -12949,6 +12949,16 @@ It then walks over every page and writes a value in it to ensure that it is used
 
 Algorithm used by the OOM: https://unix.stackexchange.com/questions/153585/how-does-the-oom-killer-decide-which-process-to-kill-first
 
+==== C multithreading
+
+Added in C11!
+
+* link:userland/c/atomic.c[]: `atomic_int` and `thrd_create`
+
+Bibliography:
+
+* https://stackoverflow.com/questions/3908031/how-to-multithread-c-code/52453354#52453354
+
 ==== GCC C extensions
 
 ===== C empty struct
@@ -12986,8 +12996,6 @@ Programs under link:userland/cpp/[] are examples of https://en.wikipedia.org/wik
 ** link:userland/cpp/template.cpp[]: basic example
 ** link:userland/cpp/template_class_with_static_member.cpp[]: https://stackoverflow.com/questions/3229883/static-member-initialization-in-a-class-template
 ** link:userland/cpp/if_constexpr.cpp[]: C++17 `if constexpr`
-*** https://stackoverflow.com/questions/12160765/if-else-at-compile-time-in-c/54647315#54647315
-*** https://stackoverflow.com/questions/37617677/implementing-a-compile-time-static-if-logic-for-different-string-types-in-a-co
 
 [[cpp-multithreading]]
 ==== C++ multithreading
@@ -12995,6 +13003,8 @@ Programs under link:userland/cpp/[] are examples of https://en.wikipedia.org/wik
 * https://en.cppreference.com/w/cpp/header/thread[`<thread>`]
 ** link:userland/cpp/count.cpp[] Exemplifies: `std::this_thread::sleep_for`
 ** link:userland/cpp/thread_hardware_concurrency.cpp[] `std::thread::hardware_concurrency`
+** link:userland/cpp/thread_get_id.cpp[] `std::thread::get_id`
+** link:userland/cpp/thread_return_value.cpp[]: how to return a value from a thread
 * https://en.cppreference.com/w/cpp/header/atomic[`<atomic>`]: <<cpp17>> 32 "Atomic operations library"
 ** link:userland/cpp/atomic.cpp[]
 
@@ -13093,6 +13103,7 @@ Bibliography: https://stackoverflow.com/questions/6988487/what-does-the-brk-syst
 The following sections are related to multithreading in userland:
 
 * language topics:
+** <<c-multithreading>>
 ** <<cpp-multithreading>>
 ** <<pthreads>>
 * ISA topics: