Assign an unique non-repeating ID to each new client.

This will be used by CLIENT KILL and is also a good way to ensure a
given client is still the same across CLIENT LIST calls.

The output of CLIENT LIST was modified to include the new ID, but this
change is considered to be backward compatible as the API does not imply
you can do positional parsing, since each filed as a different name.
New features for CLIENT KILL.
CLIENT KILL: fix closing link of the current client.
CLIENT KILL API modified.

Added a new SKIPME option that is true by default, that prevents the
client sending the command to be killed, unless SKIPME NO is sent.