resolve: when writing of private resolv.confs fails, do not remove old copies

resolve: when writing of private resolv.confs fails, do not remove old copies

All callers ignore the return value.

This is almost entirely theoretical, since writing to /run is unlikely to
fail..., but the user is almost certainly better off keeping the old copy
around and having working dns resolution with an out-of-date dns server list
than having having a dangling /etc/resolv.conf symlink.