if the user was using pop in 4.x, this defaults to 0.
a ton of code depends on it being 0, and not undefined.
alecf removed it, using it as a way to tell if we needed to migrate the prefs
or not. Instead, I'm using other prefs to do that.
see nsMsgAccountManager::ProceedWithMigration() for the logic.
7a52aab453