Improve synchronization

- Fix bugs in kDos2Errno definition
- malloc() should now be thread safe
- Fix bug in rollup.com header generator
- Fix open(O_APPEND) on the New Technology
- Fix select() on the New Technology and test it
- Work towards refactoring i/o for thread safety
- Socket reads and writes on NT now poll for signals
- Work towards i/o completion ports on the New Technology
- Make read() and write() intermittently check for signals
- Blinkenlights keyboard i/o so much better on NT w/ poll()
- You can now poll() files and sockets at the same time on NT
- Fix bug in appendr() that manifests with dlmalloc footers off

test/libc/rand/rand64_test.c

@@ -30,7 +30,7 @@
 #include "libc/sysv/consts/sig.h"
 #include "libc/testlib/testlib.h"
 
-#define THREADS 4
+#define THREADS 8
 #define ENTRIES 256
 
 volatile bool ready;
@@ -42,7 +42,9 @@ void OnChld(int sig) {
 
 int Thrasher(void *arg) {
   int i, id = (intptr_t)arg;
-  while (!ready) asm("pause");
+  while (!ready) {
+    __builtin_ia32_pause();
+  }
   for (i = 0; i < ENTRIES; ++i) {
     A[id * ENTRIES + i] = rand64();
   }