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

libc/calls/releasefd.c

@@ -17,16 +17,14 @@
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/calls/internal.h"
-#include "libc/intrin/cmpxchg.h"
-#include "libc/intrin/lockcmpxchg.h"
+#include "libc/intrin/spinlock.h"
+#include "libc/macros.internal.h"
 
 void __releasefd(int fd) {
-  int x;
-  if (!__vforked && 0 <= fd && fd < g_fds.n) {
-    bzero(g_fds.p + fd, sizeof(*g_fds.p));
-    do {
-      x = g_fds.f;
-      if (fd >= x) break;
-    } while (!_lockcmpxchg(&g_fds.f, x, fd));
+  if (0 <= fd && fd < g_fds.n) {
+    _spinlock(&__fds_lock);
+    g_fds.p[fd].kind = 0;
+    g_fds.f = MIN(fd, g_fds.f);
+    _spunlock(&__fds_lock);
   }
 }