cmd/cli: always rebootstrap when check upstream

Otherwise, network changes may not be seen on some platforms, causing
ctrld failed to recover and failing all requests.

While at it, also doing the check DNS in separate goroutine, prevent it
from blocking ctrld from notifying others that it "started". The issue
was seen when ctrld is configured as direct listener, requests are
flooded before ctrld started, causing the healtch process failed.

cmd/cli/loop.go

@@ -56,7 +56,10 @@ func (p *prog) checkDnsLoop() {
 	mainLog.Load().Debug().Msg("start checking DNS loop")
 	upstream := make(map[string]*ctrld.UpstreamConfig)
 	p.loopMu.Lock()
-	for _, uc := range p.cfg.Upstream {
+	for n, uc := range p.cfg.Upstream {
+		if p.um.isDown("upstream." + n) {
+			continue
+		}
 		uid := uc.UID()
 		p.loop[uid] = false
 		upstream[uid] = uc