ifconfig: renove global printifname variable.

[melifaro]

Summary

It was used to print the created interface name in the atexit handler.
It was calculated in the ifclonecreate() by matching old & new names.

This change alter the implementation the following way:

1. the function responsible for the interface creation (ifcreate_ioctl) updates all necessary state internally. This removes the need for the name manipulation hack in wlan_create().
2. As atexit(3) handler does not accept any parameters, explicitly store the name to print in the ifname_to_print variable read by the atexit(3) handler.