Make fixes and improvements

- Introduce __assert_disable global
- Improve strsignal() thread safety
- Make system call tracing thread safe
- Fix SO_RCVTIMEO / SO_SNDTIMEO on Windows
- Refactor DescribeFoo() functions into one place
- Fix fork() on Windows when TLS and MAP_STACK exist
- Round upwards in setsockopt(SO_RCVTIMEO) on Windows
- Disable futexes on OpenBSD which seem extremely broken
- Implement a better kludge for monotonic time on Windows

libc/sock/sendfile.c

@@ -38,7 +38,8 @@
 static textwindows int SendfileBlock(int64_t handle,
                                      struct NtOverlapped *overlapped) {
   uint32_t i, got, flags = 0;
-  if (WSAGetLastError() != kNtErrorIoPending) {
+  if (WSAGetLastError() != kNtErrorIoPending &&
+      WSAGetLastError() != WSAEINPROGRESS) {
     NTTRACE("TransmitFile failed %lm");
     return __winsockerr();
   }