Let external Eshell processes send stdout and stderr to different places

Let external Eshell processes send stdout and stderr to different places

* lisp/eshell/esh-proc.el (eshell-put-process-properties): Pass INDEX.
(eshell-gather-process-output): Create a pipe process for stderr when
stderr goes somewhere different than stdout.
(eshell-insertion-filter, eshell-sentinel): Consult
':eshell-handle-index' property.

* test/lisp/eshell/esh-proc-tests.el
(esh-proc-test/output/stdout-to-buffer)
(esh-proc-test/output/stderr-to-buffer)
(esh-proc-test/exit-status/with-stderr-pipe): New tests (bug#21605).