Put Eshell's bookkeeping data for external processes on the process object

Put Eshell's bookkeeping data for external processes on the process object

This allows tracking this information for process objects not recorded
in 'eshell-process-list', which will be useful for pipe processes for
stderr output.

* lisp/eshell/esh-proc.el (eshell-process-list): Add docstring.
(eshell-record-process-object): Only record the process object and
whether it's a subjob.
(eshell-remove-process-entry): Adapt to changes in
'eshell-record-process-object'.
(eshell-record-process-properties): New function...
(eshell-gather-process-output): ... call it.
(eshell-insertion-filter, eshell-sentinel): Use new process
properties, don't require process to be in 'eshell-process-list'.

* test/lisp/eshell/esh-proc-tests.el (esh-proc-test--output-cmd): New
variable.
(esh-proc-test--detect-pty-cmd): Add docstring.
(esh-proc-test/output/to-screen)
(esh-proc-test/output/stdout-and-stderr-to-buffer)
(esh-proc-test/exit-status/success, esh-proc-test/exit-status/failure)
(esh-proc-test/kill-process/foreground-only): New tests.
(esh-proc-test/kill-background-process): Rename to...
(esh-proc-test/kill-process/background-prompt): ... this, and use
'eshell-wait-for-subprocess' instead of 'sit-for'.