MFC r339462: make upgrade from previous FreeBSD versions less painful
and make previously working configuration like this work again:

gif_interfaces="gif0"
gifconfig_gif0="1.1.1.1 2.2.2.2"
ifconfig_gif0="inet 192.168.1.1 192.168.1.2 netmask 255.255.255.252"

PR: 204700