Moving lib/libkern/rb.* to sys/ and kern/

[Andrew Doran <ad%netbsd.org@localhost>]

Hi,

I'd like to move these. rb.h isn't available for userspace to include and
so it causes problems when embedded into data structures like vm_page. If
we use rb trees in the vm system then rb.c will always be used.

Comments?

Thanks,
Andrew