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/runtime/clone_test.c

@@ -43,6 +43,6 @@ TEST(clone, test) {
   EXPECT_NE(-1, (tid = clone(thread, stack + FRAMESIZE,
                              CLONE_VM | CLONE_FS | CLONE_FILES | CLONE_SIGHAND,
                              0, &ptid, &tls, &ctid)));
-  while ((nowl() - t) < 1 && !x) asm("pause");
+  while ((nowl() - t) < 1 && !x) __builtin_ia32_pause();
   ASSERT_EQ(42, x);
 }