Add shared memory apis to redbean

You can now do things like implement mutexes using futexes in your
redbean lua code. This provides the fastest possible inter-process
communication for your production systems when SQLite alone as ipc
or things like pipes aren't sufficient.

third_party/nsync/mu_semaphore.h

@@ -5,7 +5,7 @@
 COSMOPOLITAN_C_START_
 
 typedef struct nsync_semaphore_s_ {
-  void *sem_space[32]; /* space used by implementation */
+  void *sem_space[1]; /* [jart] reduced to 8 bytes */
 } nsync_semaphore;
 
 /* Initialize *s; the initial value is 0. */