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This implements the basic curtain-mode functionality:
* Switch out the active session on connection (if necessary--it doesn't do this if the session is at the login screen or already switched out)
* Close all connections if the session becomes active again (for example, if someone logs into it at the console).

It doesn't solve the problem of launchd starting a new instance of the server when we switch out. There will be a follow-up CL to address that.

BUG=110111


Review URL: https://chromiumcodereview.appspot.com/10823215

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@150746 0039d316-1c4b-4281-b951-d872f2087c98