routing: Bring back the ability to specify transmit interface via its name.

Some software references outgoing interfaces by specifying name instead of
index.

Use rti_ifp from rt_addrinfo if provided instead of always using
address interface when constructing nexthop.

PR: 255678
Reported by: martin.larsson2 at gmail.com

(cherry picked from commit d98954e229812eee2fa6bf97714fecbbdcc56e4c)