iplink: display the value of IFLA_PROMISCUITY

This is useful to know the 'real' status of an interface (the flag IFF_PROMISC
is exported by the kernel only when the user set it explicitly, for example it
will not be exported when a tcpdump is running).

This information will be displayed when '-details' is provided by the user.

Example:
$ ip -d l l tun10
6: tun10: <POINTOPOINT,NOARP,UP,LOWER_UP> mtu 1480 qdisc noqueue state UNKNOWN mode DEFAULT
    link/sit 10.16.0.249 peer 10.16.0.121
    sit remote 10.16.0.121 local 10.16.0.249 ttl inherit pmtudisc 6rd-prefix 2002::/16
    promiscuity 2

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
ip: change format of promiscuity display

Don't put it on separate line, keep it on line with link address.