Allow using multiple buffers in 'eshell-command'

Allow using multiple buffers in 'eshell-command'

Provide the same functionality as 'async-shell-command-buffer' but for
'eshell-command'.

Co-Authored-By: Jim Porter <jporterbugs@gmail.com>
* lisp/eshell/eshell.el (eshell-command-async-buffer): New option...
(eshell-command): ... use it.

* lisp/eshell/esh-proc.el (eshell-sentinel): Check for buffer liveness
in 'finish-io'.

* test/lisp/eshell/eshell-tests.el
(eshell-test/eshell-command/output-buffer/async-kill): New test.

* etc/NEWS: Announce this change (bug#71554).

(cherry picked from commit 7f631a3e2aca97e95b8659c902c25ab21f084e08)