Fix activation hook inconsistency

[nacin]

The activation hook fires whenever a plugin is updated through the admin, but not during bulk updates.

We need to resolve this inconsistency, and either A) fire activation hooks for bulk updates, or B) stop activation hooks from firing for single updates.

I think I suggest (A), despite the backwards compatibility issue. It's unintuitive for an activation hook to be fired on update, and additionally it is not reliable due to manual updates (including SVN). As it is, we silence deactivation hooks on update.

Related, a register_upgrade_hook() ticket #14912, that I believe should be wontfix'd. Schema updates require special handling at the plugin level IMO for now.