Show 'external' link mode in output

Show 'external' link mode in output

Recently `external` support was added to the tunnel drivers, but there is no way
to introspect this from userspace. This adds support for that.

Now `ip -details link` shows it:

```
7: tunl60@NONE: <NOARP> mtu 1452 qdisc noop state DOWN mode DEFAULT group
default qlen 1
    link/tunnel6 :: brd :: promiscuity 0
    ip6tnl external any remote :: local :: encaplimit 0 hoplimit 0 tclass 0x00 flowlabel 0x00000 (flowinfo 0x00000000) addrgenmode eui64 numtxqueues 1 numrxqueues 1 gso_max_size 65536 gso_max_segs 65535
```

Signed-off-by: Phil Dibowitz <phil@ipom.com>