bug#8968: arc-mode 7z writing support

[Juri Linkov]

> Some users prefer using 7z even for operations on zip archives.
>
> This patch provides the right default values for them in case
> 7z is installed on the system instead of zip/unzip.
>
> For searching the available programs it uses exactly the same logic
> as already is implemented in `archive-zip-extract' (i.e. first try
> to find zip/unzip, then 7z, and finally pkzip/pkunzip):

This is installed as well.

As reported in http://thread.gmane.org/gmane.emacs.devel/136578/focus=136670
on cygwin the program name is "p7zip".  But I can't confirm this fact,
so I'm closing this feature request at its current state with the
program name "7z".