Module enabling/initialization hook for initial setup

A module often has some one-time setup initialization to do, some of which can be easily automated (e.g., set a system variable), some of which may require site admin input. But there's no available method to detect and act on module enabling.

So the attached patch implements one possible approach: a new _enable() hook, that would be called when a module is enabled.

I suppose an alternate approach would be simply to redirect to the module's setting's page, if available; but, because multiple modules can be enabled at the same time, that might be impractical.

I know, three days before code freeze is not the best time to be bringing up new proposed hooks! I'll be happy to wait until next cycle to work the idea through.