[source synchronization hooks] Fix/enhance system source hooks
They are broken if launched during repository initialization (this was not the
case yet, but will be soon...). Add additional checks to ensure one doesn't try
to store system source config in the database, as it will be ignored in favor of
the "sources" file.
usr/lib/python2.4/site-packages/cubicweb/
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