Update French translation
Add polkit rule so members of group "kalu" don't need auth to run a sysupgrade

This obviously only affects sysupgrade ran through kalu's updater. The group
name can be set on configure. It's also possible to just copy the file to
/etc/polkit-1/rules.d to overwrite it at "runtime".
Fix man page (remove obsolete paragraph)
Set user-agent via environment variable HTTP_USER_AGENT; Fix #5

Setting a user-agent isn't a bad idea, it also fix issue with some server/repo
where lack of one resulted in a download error (HTTP 406).