[th/dns-rc-manager-auto] dns: add new "rc-manager=auto" mode

Add a new main.rc-manager=auto setting, that favours to use systemd-resolved (and not touch "/etc/resolv.conf" but configure it via D-Bus), or falls back to resolvconf/netconfig binaries if they are installed. As final fallback use "symlink", like before.

Suggested-by: Jason A. Donenfeld Jason@zx2c4.com