Runtime kernel checking

It's not at all obvious that one needs a newer kernel to use rr; none of the wiki pages document the restriction on the kernel, and one needs to read the presentation meant for potential rr developers to find out what kernel versions are suitable.

Even when the documentation gets updated, who reads that?  Make rr check that it's running on a suitable kernel and helpfully inform the user what's going on, rather than obscure `perror` messages or similar.