Add debug instrumentation for Eshell process management

Add debug instrumentation for Eshell process management

* lisp/eshell/esh-util.el (eshell-debug-command)
* lisp/eshell/em-basic.el (eshell/eshell-debug)
(pcomplete/eshell-mode/eshell-debug): Add 'process' type.

* lisp/eshell/esh-proc.el (eshell-gather-process-output)
(eshell-interactive-process-filter, eshell-insertion-filter)
(eshell-sentinel): Call 'eshell-debug-command'.

* test/lisp/eshell/eshell-tests-helpers.el (with-temp-eshell): Add
'process' to 'eshell-debug-command'
(eshell-get-debug-logs): New function...
(eshell-match-command-output, eshell-command-result-equal): ... use
it.

* doc/misc/eshell.texi (Built-ins): Mention "process" debug type.