Collect all processes in an Eshell pipeline, not just the head and tail

Collect all processes in an Eshell pipeline, not just the head and tail

This has the extra benefit that Eshell now only considers a pipeline
to be done when *all* of its processes are done (previously, it
checked only the last one in the pipeline).

* lisp/eshell/esh-util.el (eshell-process-pair-p)
(eshell-make-process-pair): Rename to...
(eshell-process-list-p, eshell-make-process-list): ... these, and
handle lists of processes.  Update callers.

* lisp/eshell/esh-cmd.el (eshell-head-process): Use 'car'.
(eshell-tail-process): Get the last element of the list.
(eshell-do-pipelines): Return a list of all processes in the pipeline.
(eshell-do-pipelines-synchronously): Return the result of the first
command (usually t or nil).
(eshell-execute-pipeline): Simplify.
(eshell-do-eval): Pass all processes to 'eshell/wait'.