* #6630 : don't make DBusManager a singleton

Instead it is part of Manager which is already itself a singleton
When the client requests termination of the daemon, the DBusManager instance
is not destructed, so we avoid using already freed memory

make Manager::(un)serialize static functions, they don't depend on the instance state
No need to check for the NULLity of _dbus anymore

Remove a useless argument to initConfigFile