ss: Add support for retrieving SELinux contexts

ss: Add support for retrieving SELinux contexts

The process SELinux contexts can be added to the output using the -Z
option. Using the -z option will show the process and socket contexts (see
the man page for details).
For netlink sockets: if valid process show process context, if pid = 0
show kernel initial context, if unknown show "unavailable".

Signed-off-by: Richard Haines <richard_c_haines@btinternet.com>