routing: install prefix and loopback routes using new nhop-based KPI.

Construct the desired hexthops directly instead of using the
"translation" layer in form of filling rt_addrinfo data.
Simplify V_rt_add_addr_allfibs handling by using recently-added
rib_copy_route() to propagate the routes to the non-primary address
fibs.

MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D36166