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/runtime/mremap.c

@@ -68,6 +68,10 @@ static bool MustMoveMap(intptr_t y, size_t j) {
  * @param q is new address
  */
 void *mremap(void *p, size_t n, size_t m, int f, ... /* void *q */) {
+  enosys();
+  return MAP_FAILED;
+
+#if 0
   va_list va;
   void *res, *q;
   if (f & MREMAP_FIXED) {
@@ -83,7 +87,6 @@ void *mremap(void *p, size_t n, size_t m, int f, ... /* void *q */) {
          DescribeRemapFlags(f), q, res);
   return res;
 
-#if 0
   // TODO(jart): perhaps some day?
   //             probably not a big perf gain at this point :|
   size_t i, j, k;