ss: add AF_VSOCK support

ss: add AF_VSOCK support

The AF_VSOCK address family is a host<->guest communications channel
supported by VMware, KVM, and Hyper-V.  Initial VMware support was
released in Linux 3.9 in 2013 and transports for other hypervisors were
added later.

AF_VSOCK addresses are <u32 cid, u32 port> tuples.  The 32-bit cid
integer is comparable to an IP address.  AF_VSOCK ports work like
TCP/UDP ports.

Both SOCK_STREAM and SOCK_DGRAM socket types are available.

This patch adds AF_VSOCK support to ss(8) so that sockets can be
observed.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>