Fix another race condition when waiting for Eshell processes

Fix another race condition when waiting for Eshell processes

When checking if the other processes in our pipeline are "alive", we
also need to check whether their sentinels are finished.  Otherwise,
we might proceed with command evaluation while one of the other
processes is still cleaning up.

* lisp/eshell/esh-proc.el (eshell-process-active-p): New function...
(eshell-wait-for-process)
* lisp/eshell/esh-cmd.el (eshell-resume-command): ... use it.