Allow Dired to dereference symbolic links when copying

Allow Dired to dereference symbolic links when copying

* doc/emacs/dired.texi (Operating on Files): Mention the new
defcustom (bug#25075).
* lisp/dired-aux.el (dired-do-copy): Invert the value of
`dired-copy-dereference' in lexical scope when prefix argument is
'(4). Update function documentation for the new defcustom.

* lisp/dired-aux.el (dired-copy-file): Use `dired-copy-dereference' as
the `dereference' argument to `dired-copy-file-recursive'.

* lisp/dired-aux.el (dired-copy-file-recursive): Add new optional
argument `dereference'.

* lisp/dired.el (dired-copy-dereference): New defcustom, defaults to
nil.