arp: fix adding proxy entries for P2P interfaces

The old rtsock implementation used in-kernel workaround to do so.
When route to specified destination address used P2P interface, the
kernel did the search with ifa_ifwithnet() for most suitable network
and then add proxy entry to this interface.

Use similar approach with netlink implementation. We already have
get_ether_addr() function that does almost the same thing as
ifa_ifwithnet(). Use it when we find that destination route uses
P2P interface and then try to guess suitable interface. This should
fix the use of netlink-based arp(8) in mpd5.

Rename get_ether_addr() to get_ifinfo(), since now it is used to find
only ifindex in case when hwaddr is specified by user.
Also make set_nl() and delete_nl() prototype similar to rtsock.
And allow '-i' to be used with '-S', since we already allow the same
for '-s'.

PR: 290221
Reported by: eugen
Reviewed by: eugen
Differential Revision: https://reviews.freebsd.org/D53113

(cherry picked from commit c26d6bf9da013e839d9a696746ea1741569e619e)