Add 'eshell-duplicate-handles' to return a copy of file handles

Add 'eshell-duplicate-handles' to return a copy of file handles

* lisp/eshell/esh-io.el (eshell-create-handles): Support creating with
multiple targets for stdout and/or stderr.  Make the targets for a
handle always be a list, and store whether the targets are the default
in a separate 'default' field.
(eshell-protect-handles, eshell-close-handles)
(eshell-copy-output-handle, eshell-interactive-output-p)
(eshell-output-object): Update for changes in 'eshell-create-handles'.
(eshell-duplicate-handles, eshell-get-targets): New functions.

* lisp/eshell/esh-cmd.el (eshell-copy-handles): Rename and alias to...
(eshell-with-copied-handles): ... this function, and use
'eshell-duplicate-handles'.
(eshell-execute-pipeline): Use 'eshell-duplicate-handles'.