Decompose arp_ifinit() into arp_add_ifa_lle() and arp_announce_ifaddr().
Rename arp_ifinit2() into arp_announce_ifaddr().

Eliminate zeroing ifa_rtrequest: it was used for calling arp_rtrequest()
which was responsible for handling route cloning requests. It became
obsolete since r186119 (L2/L3 split).