nspawn: suppress warning when /etc/resolv.conf is a valid symlink

nspawn: suppress warning when /etc/resolv.conf is a valid symlink

In such a case let's suppress the warning (downgrade to LOG_DEBUG),
under the assumption that the user has no config file to update in its
place, but a symlink that points to something like resolved's
automatically managed resolve.conf file.

While we are at it, also stop complaining if we cannot write /etc/resolv.conf
due to a read-only disk, given that there's little we could do about it.