Add a "banner" command to the CLI interface.

Add a "banner" command to the CLI interface.

Add a parameter "cli_banner" (default on) which injects an implicit
"banner" CLI command on all cli connections when opened.

The net result is that you get a CLI response when you connect to
the CLI ports:

	200 193
	-----------------------------
	Varnish HTTP accelerator CLI.
	-----------------------------
	Type 'help' for command list.
	Type 'quit' to close CLI session.
	Type 'start' to launch worker process.

Presently the contents of the CLI response is "undefined", in the
sense that you should not programatically depend on anything besides
the "200" reply code.

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@3762 d4fa192b-c00b-0410-8231-f00ffab90ce4