Consolidates the logout code into Bugzilla::Auth::CGI, and provides
simple front-end wrappers in Bugzilla.pm for use in the CGIs we have.
r=bbaetz, joel; a=justdave.

Adds a set of constants to the logout() API which allow specifying "how
much" we should log out -- all sessions, the current session, or all
sessions but the current one.

Fixes callsites to use this new API; cleans and documents things a
bit while we're at it. Part I in the great COOKIE apocalypse.