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/writev-nt.c

@@ -27,7 +27,7 @@ textwindows ssize_t sys_writev_nt(int fd, const struct iovec *iov, int iovlen) {
     case kFdConsole:
       return sys_write_nt(fd, iov, iovlen, -1);
     case kFdSocket:
-      return weaken(sys_sendto_nt)(fd, iov, iovlen, 0, NULL, 0);
+      return weaken(sys_send_nt)(fd, iov, iovlen, 0);
     default:
       return ebadf();
   }