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/proc/fork-nt.c

@@ -401,7 +401,7 @@ textwindows int sys_fork_nt(uint32_t dwCreationFlags) {
     }
   }
   if (rc == -1) {
-    __proc_free(proc);
+    dll_make_first(&__proc.free, &proc->elem);
   }
   ftrace_enabled(+1);
   strace_enabled(+1);