Put more thought into i/o polyfills

wait4() is now solid enough to run `make -j100` on Windows. You can now
use MSG_DONTWAIT on Windows. There was a handle leak in accept() that's
been fixed. Our WIN32 overlapped i/o code has been simplified. Priority
class now inherits into subprocesses, so the verynice command will work
and the signal mask will now be inherited by execve() and posix_spawn()

libc/calls/ntspawn.c

@@ -126,7 +126,8 @@ textwindows int ntspawn(
           if (CreateProcess(sb->path, sb->cmdline, 0, 0, true,
                             dwCreationFlags | kNtCreateUnicodeEnvironment |
                                 kNtExtendedStartupinfoPresent |
-                                kNtInheritParentAffinity,
+                                kNtInheritParentAffinity |
+                                GetPriorityClass(GetCurrentProcess()),
                             sb->envblock, opt_lpCurrentDirectory,
                             &info.StartupInfo, opt_out_lpProcessInformation)) {
             rc = 0;