A configuration like the following:

7: tun6rd: <NOARP,UP,LOWER_UP> mtu 1280
        link/sit 10.11.12.13 brd 0.0.0.0
        inet 1.2.3.4/24 scope global tun6rd
        inet6 2014:1001:a0b:c0d::1/32 scope global
        inet6 ::10.11.12.13/128 scope global
9: ppp0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1496
        link/ppp
        inet 192.168.101.234 peer 192.168.102.234/32 scope global ppp0
        inet 10.20.30.40/24 scope global ppp0
        inet6 2014:1002::1/64 scope global
11: tun0@NONE: <POINTOPOINT,NOARP,UP,LOWER_UP> mtu 1480
        link/ipip 192.168.202.34 peer 192.168.202.69
        inet 192.168.10.1/24 scope global tunnel0
        inet6 2014:1003::1/64 scope global

will be handled like below.

"tun6rd": flags "up", ifindex 7, mtu 1280
        hardware address ""
        interface address "1.2.3.4/24"
        interface address "2014:1001:a0b:c0d::1/32"
        interface address "::a0b:c0d/128"
"ppp0": flags "up|pointtopoint|multicast", ifindex 9, mtu 1496
        hardware address ""
        interface address "192.168.101.234/32"
        interface address "10.20.30.40/24"
        interface address "2014:1002::1/64"
"tun0": flags "up|pointtopoint", ifindex 11, mtu 1480
        hardware address ""
        interface address "192.168.10.1/24"
        interface address "2014:1003::1/64"

Fixes #6433.
Update #4839

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/57700043