When installing a new ARP entry via 'arp -S', lla_lookup() will
either find an existing entry, or allocate a new one. In the latter
case an entry would have flags, that were supplied as argument to
lla_lookup(). In case of an existing entry, flags aren't modified.

This lead to losing LLE_PUB and/or LLE_PROXY flags.

We should apply these flags either in lla_rt_output() or in the
in.c:in_lltable_lookup(). It seems to me that lla_rt_output() is
a more correct choice.

PR:           kern/[[ https://bugs.freebsd.org/148784 | 148784 ]], kern/146539