cmd/cli: fix issue with editing /etc/resolv.conf directly on Darwin

On Darwin, modifying /etc/resolv.conf directly does not change interface
network settings. Thus the networksetup command uses to set DNS does not
do anything.

To fix this, after setting DNS using networksetup, re-check the content
of /etc/resolv.conf file to see if the nameservers are what we expected.
Otherwise, re-generate the file with proper nameservers.

cmd/cli/prog.go

@@ -651,6 +651,8 @@ func (p *prog) dnsWatchdog(iface *net.Interface, nameservers []string, allIfaces
 					if dnsChanged(i, ns) {
 						if err := setDnsIgnoreUnusableInterface(i, nameservers); err != nil {
 							mainLog.Load().Error().Err(err).Str("iface", i.Name).Msgf("could not re-apply DNS settings")
+						} else {
+							mainLog.Load().Debug().Msgf("re-applying DNS for interface %q successfully", i.Name)
 						}
 					}
 					return nil