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.

libc/zipos/open.c

@@ -54,7 +54,7 @@ static void *__zipos_mmap(size_t mapsize) {
   maptotal += mapsize;
   start = (char *)kMemtrackZiposStart;
   if (!mapend) mapend = start;
-  mapend = _extend(start, maptotal, mapend,
+  mapend = _extend(start, maptotal, mapend, MAP_PRIVATE,
                    kMemtrackZiposStart + kMemtrackZiposSize);
   return start + offset;
 }