When executing an Eshell pipeline, send input to the first process

When executing an Eshell pipeline, send input to the first process

Previously, input was sent to the last process in the pipeline,
resulting in unexpected behavior when running commands like
'tr a-z A-Z | rev'.

* lisp/eshell/esh-util.el (eshell-process-pair-p)
(eshell-make-process-pair): New functions.

* lisp/eshell/esh-cmd.el (eshell-last-async-proc): Rename to...
(eshell-last-async-procs): ... this, and store a pair of processes.
(eshell-interactive-process): Replace with...
(eshell-interactive-process-p, eshell-head-process)
(eshell-tail-process): ... these.
(eshell-cmd-initialize): Set 'eshell-last-async-procs'.
(eshell-do-pipelines): Set 'headproc'.
(eshell-execute-pipeline): Return 'headproc' and 'tailproc'.
(eshell-resume-eval): Use 'eshell-last-async-procs'.
(eshell-do-eval): Make sure we work with a pair of processes.

* lisp/eshell/esh-proc.el (eshell-send-eof-to-process): Move from
here...
* lisp/eshell/esh-mode.el (eshell-send-eof-to-process): ... to here,
and only send EOF to the head process.

* lisp/eshell/em-cmpl.el (eshell-complete-parse-arguments)
* lisp/eshell/esh-mode.el (eshell-intercept-commands)
(eshell-watch-for-password-prompt):
Use 'eshell-interactive-process-p'.

* lisp/eshell/em-rebind.el (eshell-delchar-or-maybe-eof)
* lisp/eshell/em-term.el (eshell-term-send-raw-string)
* lisp/eshell/esh-mode.el (eshell-self-insert-command)
(eshell-send-input, eshell-send-invisible):
Use 'eshell-head-process'.

* lisp/eshell/esh-cmd.el (eshell-as-subcommand):
Use 'eshell-tail-process'.

* lisp/eshell/eshell.el (eshell-command):
* test/lisp/eshell/eshell-tests-helpers.el
(eshell-wait-for-subprocess):
Use 'eshell-interactive-process-p' and 'eshell-tail-process'.

* test/lisp/eshell/eshell-tests.el (eshell-test/pipe-headproc-stdin):
New test.