The EVENTHANDLER mechanism provides a way for kernel subsystems to register interest in kernel events and have their callback functions invoked when these events occur.

Still with minimal stack, no interrupt support, etc.

Simpler than phys_cast sometimes.

I've also added an osv/ prefix to make sure we don't clash with system headers.

We probably should either adopt this globally or drop it, but I fear we'll run into
problems with the latter approach.