Deprecating --username/--password and login/logout in favor of new -auth...

Deprecating --username/--password and login/logout in favor of new -auth option passing BASIC authentication to /cli endpoint.
Simpler, does not rely on Remoting, allows use of API tokens, and bypasses JENKINS-12543.
(You could actually do this before but only by embedding userinfo in the -s URL, especially awkward for usernames containing @.)