quit: better handling of `save` for saving package list

Ideally, the call to `savepkglist` should be made if `save = "yes"`, but also interactively if `save = "default"` or `save = "ask"`. The problem is that  `q` is an internal function, not to be modified...

A proper mechanism would then allow `savePkg = FALSE` or a similar mechanism to make it more generic.
