Improve Eshell's behavior when waiting for processes

Improve Eshell's behavior when waiting for processes

This has a few benefits.  First, it fixes a race condition when killing
old processes in 'eshell-command'.  Second, the "wait" built-in command
is now more useful.  Finally, killing processes when exiting Eshell (via
'eshell-round-robin-kill') should be much faster.

* lisp/eshell/esh-proc.el (esh-opt): Require.
(eshell-wait-for-process): Make obsolete in favor of...
(eshell-wait-for-processes): ... this.  Accept a timeout and support
PIDs.  Update callers.
(eshell/wait): New implementation accepting -t/--timeout.
(eshell-round-robin-kill): Use 'eshell-wait-for-processes'.

* lisp/eshell/eshell.el (eshell-command): Use 'eshell-round-robin-kill'.

* doc/misc/eshell.texi (List of Built-ins): Document the new "wait"
behavior.

* etc/NEWS: Announce this change.

(cherry picked from commit 8e46f44ea0eb761e24beda8c5cdbc8fcca87307a)