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/fdatasync-nt.c

@@ -21,6 +21,7 @@
 #include "libc/sysv/errfuns.h"
 
 textwindows int sys_fdatasync_nt(int fd) {
+  // TODO(jart): what should we do with worker pipes?
   if (!__isfdkind(fd, kFdFile)) return ebadf();
   return FlushFileBuffers(g_fds.p[fd].handle) ? 0 : -1;
 }