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/CMakeLists.txt

@@ -100,6 +100,7 @@ set(KERNEL_SOURCES
     Syscalls/clock.cpp
     Syscalls/debug.cpp
     Syscalls/disown.cpp
+    Syscalls/dup2.cpp
     Syscalls/execve.cpp
     Syscalls/exit.cpp
     Syscalls/fcntl.cpp