Add a new macro to simplify parsing temporary Eshell command strings

Add a new macro to simplify parsing temporary Eshell command strings

This abstracts out the somewhat-unusual "insert&delete" logic in
'eshell-parse-command' so that it can be used elsewhere, and also
ensures that the deletion occurs even if an an error occurs.

* lisp/eshell/esh-cmd.el (eshell-with-temp-command): New macro.
(eshell-parse-command): Use it.