iproute: Document the "ip link add index IDX" possibility

Signed-off-by: Pavel Emelyanov <xemul@paralles.com>
ss: handle seqpacket type of unix domain socket

ss didn't distignish seqpacket type from dgram type.
With this patch ss can distignish it.

 $ misc/ss -x -a | grep seq
 u_seq  LISTEN     0      128    /run/udev/control 10966                 * 0
 u_seq  ESTAB      0      0                    * 115103                * 115104
 u_seq  ESTAB      0      0                    * 115104                * 115103

Signed-off-by: Masatake YAMATO <yamato@redhat.com>
ss: enable query by type in unix domain related socket

This patch enables -A unix_stream, -A unix_dgram and
-A unix_seqpacket option even if ss gets socket information
via netlink.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>
ss: add unix_seqpacket to the help message and the man page

Signed-off-by: Masatake YAMATO <yamato@redhat.com>