Improve handling of Eshell "for" loops

Improve handling of Eshell "for" loops

This fixes some errors with more-complex string forms, and also allows
iterating over sequences other than just lists.

* lisp/eshell/esh-cmd.el (eshell-for-iterate): New function...
(eshell-rewrite-for-command): ... use it.

* test/lisp/eshell/esh-cmd-tests.el (esh-cmd-test/for-loop): Test
multiple values.
(esh-cmd-test/for-loop-string, esh-cmd-test/for-loop-vector): New tests.
(esh-cmd-test/for-loop-mixed-args): Rename.

* test/lisp/eshell/esh-proc-tests.el
(esh-proc-test/sentinel/change-buffer): Make sure all the processes get
cleaned up.

(cherry picked from commit 08d5994b435c119bde8b8ac8f7152810d1814d1e)