Always removing wep_tx_keyidx will cause wpa_supplicant.conf being
written even if nothing has been changed. Now it will be removed only
when wep is not used anymore.
nm_utils_hwaddr_ntoa() and nm_utils_hwaddr_aton() are like
ether_ntoa()/ether_aton(), but handle IPoIB too.
nm_utils_hwaddr_atoba() is like _aton() but returns a GByteArray,
since that's what's wanted in many places.
Also remove nm_ether_ntop() and replace uses of it with
nm_utils_hwaddr_ntoa().