RFC: renameat(): Add a RENAME_REMOVE flag to unlink hardlinks

If 'a' and 'b' are hardlinks, then the command `mv a b & mv b a`
can result in both being removed as mv needs to emulate the
move with an unlink of the source file.  This can only be done
without races in the kernel and so mv was recently changed
to not allow this operation at all.  mv could safely reintroduce
this feature by leveraging a new flag for renameat() for which
an illustrative/untested patch is attached.

thanks,
Pádraig.