Kernel+LibC+UserspaceEmulator: Bring back sys$dup2()

This is racy in userspace and non-racy in kernelspace so let's keep
it in kernelspace.

The behavior change where CLOEXEC is preserved when dup2() is called
with (old_fd == new_fd) was good though, let's keep that.

Kernel/API/Syscall.h

@@ -86,6 +86,7 @@ namespace Kernel {
     S(getpgrp)                \
     S(fork)                   \
     S(execve)                 \
+    S(dup2)                   \
     S(sigaction)              \
     S(umask)                  \
     S(getgroups)              \