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.
git-svn-id: svn://10.0.0.236/trunk@154327 18797224-902f-48f8-a5cc-f745e15eee43
