[py3k] introduce cubicweb._ (related to #7589459) The end goal is to stop relying on '_' being in the builtin scope. In the mean time, deprecate the builtin '_' so that consumers are warned.

etc/init.d
etc/cubicweb.d
etc/bash_completion.d
usr/bin
usr/share/doc/cubicweb-ctl
var/log/cubicweb
var/lib/cubicweb/backup
var/lib/cubicweb/instances