bug#10598: 24.0.92; run dired-do-async-shell-command on multiple files individually

[Juri Linkov]

> Am I misunderstanding the docstrings of dired-do-shell-command and
> dired-do-async-shell-command (which are a bit confusing for me
> concerning how to run a shell command on multiple files) or is there
> a bug in the code?

I think `dired-shell-stuff-it' should be improved to translate
`gv foo.ps&;gv bar.ps&' into `gv foo.ps&gv bar.ps&' or into
`gv foo.ps;gv bar.ps&' depending on whether you want to run
asynchronously both at the same time or sequentially
(perhaps a new option should define that).