Simplify dom_<rtattach|rtdetach>.

Remove unused arguments from dom_rtattach/dom_rtdetach functions and make

them return/accept 'struct rib_head' instead of 'void **'.

Declare inet/inet6 implementations in the relevant _var.h headers similar

to domifattach / domifdetach.

Add rib_subscribe_internal() function to accept subscriptions to the rnh

directly.

Differential Revision: https://reviews.freebsd.org/D26053