all: use strings.Cut even more

Change-Id: I943ce72c6f339589235bddbe10d07799c4e37979
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/filter/filter_test.go

@@ -676,7 +676,7 @@ func pfx(strs ...string) (ret []netaddr.IPPrefix) {
 
 func nets(nets ...string) (ret []netaddr.IPPrefix) {
 	for _, s := range nets {
-		if i := strings.IndexByte(s, '/'); i == -1 {
+		if !strings.Contains(s, "/") {
 			ip, err := netaddr.ParseIP(s)
 			if err != nil {
 				panic(err)

wgengine/magicsock/debughttp.go

@@ -188,8 +188,8 @@ func peerDebugName(p *tailcfg.Node) string {
 		return ""
 	}
 	n := p.Name
-	if i := strings.Index(n, "."); i != -1 {
-		return n[:i]
+	if base, _, ok := strings.Cut(n, "."); ok {
+		return base
 	}
 	return p.Hostinfo.Hostname()
 }

wgengine/magicsock/magicsock.go

@@ -905,9 +905,7 @@ func (c *Conn) Ping(peer *tailcfg.Node, res *ipnstate.PingResult, cb func(*ipnst
 	if res.NodeName == "" {
 		res.NodeName = peer.Hostinfo.Hostname() // else hostname
 	} else {
-		if i := strings.Index(res.NodeName, "."); i != -1 {
-			res.NodeName = res.NodeName[:i]
-		}
+		res.NodeName, _, _ = strings.Cut(res.NodeName, ".")
 	}
 
 	ep, ok := c.peerMap.endpointForNodeKey(peer.Key)