Add 'eshell-special-ref-alist' to allow extending Eshell special refs

Add 'eshell-special-ref-alist' to allow extending Eshell special refs

* lisp/eshell/esh-cmd.el (eshell--region-p, eshell-with-temp-command):
Move to...
* lisp/eshell/esh-util.el (eshell--region-p)
(eshell-with-temp-command): ... here.

* lisp/eshell/em-cmpl.el (eshell-complete-parse-arguments): Fix edge
case when 'end' is at beginning of (possibly-narrowed) buffer.

* lisp/eshell/esh-arg.el (eshell-special-ref-alist) New variable...
(eshell-special-ref-default): ... New option...
(eshell--special-ref-function): ... New function...
(eshell-parse-special-reference): ... use them.
(eshell-insert-special-reference): New function.
(eshell-complete-special-reference): Reimplement to use a nested call
to Pcomplete.
(eshell-complete-buffer-ref): New function.

* lisp/eshell/esh-proc.el (eshell-proc-initialize): Add "process"
special ref type here.
(eshell-complete-process-ref): New function.

* doc/misc/eshell.texi (Bugs and ideas): Remove now-implemented idea.