all: use strs.CutPrefix and strs.CutSuffix more

Updates places where we use HasPrefix + TrimPrefix to use the combined
function.

Updates #5309

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

net/dns/resolvconffile/resolvconffile.go

@@ -21,6 +21,7 @@ import (
 	"strings"
 
 	"tailscale.com/util/dnsname"
+	"tailscale.com/util/strs"
 )
 
 // Path is the canonical location of resolv.conf.
@@ -68,8 +69,7 @@ func Parse(r io.Reader) (*Config, error) {
 		line, _, _ = strings.Cut(line, "#") // remove any comments
 		line = strings.TrimSpace(line)
 
-		if strings.HasPrefix(line, "nameserver") {
-			s := strings.TrimPrefix(line, "nameserver")
+		if s, ok := strs.CutPrefix(line, "nameserver"); ok {
 			nameserver := strings.TrimSpace(s)
 			if len(nameserver) == len(s) {
 				return nil, fmt.Errorf("missing space after \"nameserver\" in %q", line)
@@ -82,8 +82,7 @@ func Parse(r io.Reader) (*Config, error) {
 			continue
 		}
 
-		if strings.HasPrefix(line, "search") {
-			s := strings.TrimPrefix(line, "search")
+		if s, ok := strs.CutPrefix(line, "search"); ok {
 			domain := strings.TrimSpace(s)
 			if len(domain) == len(s) {
 				// No leading space?!

net/dns/resolver/tsdns.go

@@ -34,6 +34,7 @@ import (
 	"tailscale.com/util/clientmetric"
 	"tailscale.com/util/cloudenv"
 	"tailscale.com/util/dnsname"
+	"tailscale.com/util/strs"
 	"tailscale.com/wgengine/monitor"
 )
 
@@ -1215,8 +1216,7 @@ func (r *Resolver) respond(query []byte) ([]byte, error) {
 // unARPA maps from "4.4.8.8.in-addr.arpa." to "8.8.4.4", etc.
 func unARPA(a string) (ipStr string, ok bool) {
 	const suf4 = ".in-addr.arpa."
-	if strings.HasSuffix(a, suf4) {
-		s := strings.TrimSuffix(a, suf4)
+	if s, ok := strs.CutSuffix(a, suf4); ok {
 		// Parse and reverse octets.
 		ip, err := netip.ParseAddr(s)
 		if err != nil || !ip.Is4() {