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/openat-metal.c

@@ -29,12 +29,13 @@ int sys_openat_metal(int dirfd, const char *file, int flags, unsigned mode) {
   struct MetalFile *state;
   if (strcmp(file, "ape.com")) return enoent();
   if (!weaken(calloc)) return enomem();
-  if ((fd = __reservefd()) == -1) return -1;
+  if ((fd = __reservefd(-1)) == -1) return -1;
   state = weaken(calloc)(1, sizeof(struct MetalFile));
   state->base = (char *)_base;
   state->size = _end - _base;
   g_fds.p[fd].kind = kFdFile;
   g_fds.p[fd].flags = flags;
+  g_fds.p[fd].mode = mode;
   g_fds.p[fd].handle = (intptr_t)state;
   return fd;
 }