[Nut-upsdev] configure: added --with-group

[Arjen de Korte]

I just committed the --with-group option for configure. One thing is not
quite what I like, but I have no idea how to script this in autoconf.
Currently both RUN_AS_USER and RUN_AS_GROUP have the defaults "nobody"
and "nogroup" respectively. I think this really should be changed to the
following:

- if neither --with-user, nor --with-group are specified, we apply the
defaults "nobody" and "nogroup" (or whatever are sane defaults)

- if both --with-user and --with-group are specified, use the values
provided

- if either --with-user or --with-group is specified (but not both) this
should result in an error message, stating that specifying either one
means that the other is mandatory

The above changes will make packagers aware that they need to add the
--with-group option in order for the hotplug/udev scripts to be of any
use and that they have to think about which group to use. Using the
username as default groupname, will lead to the same confusion we have
already.

Best regards, Arjen