ss: Introduce -T, --threads option

ss: Introduce -T, --threads option

The -p, -Z and -z options only show process (thread group leader)
information.  For example, if the thread group leader has exited, but
another thread in the group is still using a socket, ss -[pZz] does not
show it.

Add a new option, -T (--threads), to show thread information.  It implies
the -p option.  For example, imagine process A and thread B (in the same
group) using the same socket.  ss -p only shows A:

  $ ss -ltp "sport = 1234"
  State   Recv-Q  Send-Q  Local Address:Port      Peer Address:Port       Process
  LISTEN  0       100           0.0.0.0:1234           0.0.0.0:*           users:(("test",pid=2932547,fd=3))

ss -T shows A and B:

  $ ss -ltT "sport = 1234"
  State   Recv-Q  Send-Q  Local Address:Port      Peer Address:Port       Process
  LISTEN  0       100           0.0.0.0:1234           0.0.0.0:*           users:(("test",pid=2932547,tid=2932548,fd=3),("test",pid=2932547,tid=2932547,fd=3))

If -T is used, -Z and -z also show SELinux contexts for threads.

Rename some variables (from "process" to "task", for example) since we
use them for both processes and threads.

Signed-off-by: Peilin Ye <peilin.ye@bytedance.com>
Signed-off-by: David Ahern <dsahern@kernel.org>