lstopo-draw: uniformize the management of the text
lstopo-draw: uniformize the management of the text

Always use lstopo_obj_snprintf(), move obj->name uses there.
Don't hardwire the text width, compute it based on the output of
lstopo_obj_snprintf() so that we can make it more flexible later.

shekkbuilder/hwloc,shekkbuilder/hwloc,shekkbuilder/hwloc,shekkbuilder/hwloc