When waiting for a process in Eshell, consult its status

When waiting for a process in Eshell, consult its status

This should be functionally the same as the previous implementation in
most cases (which consulted its membership in 'eshell-process-list'),
but is more flexible.  It's now possible to wait for processes that
aren't in 'eshell-process-list'.

Additionally, use 'process-live-p' instead of examining
'process-status' in a few places.  This is simpler, and a bit more
correct too for certain types of processes (though it likely doesn't
matter in practice).

* lisp/eshell/esh-io.el (eshell-close-target)
(eshell-output-object-to-target)
* lisp/eshell/esh-proc.el (eshell-process-interact): Use
'process-live-p'.
(eshell-wait-for-process): Use 'process-live-p' and remove reference
to 'eshell-process-list'.