various: add golangci-lint, fix issues (#7905)

This adds an initial and intentionally minimal configuration for
golang-ci, fixes the issues reported, and adds a GitHub Action to check
new pull requests against this linter configuration.

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I8f38fbc315836a19a094d0d3e986758b9313f163

net/dns/manager_tcp_test.go

@@ -108,7 +108,7 @@ func TestDNSOverTCP(t *testing.T) {
 		"bradfitz.ts.com.": "2.3.4.5",
 	}
 
-	for domain, _ := range wantResults {
+	for domain := range wantResults {
 		b := mkDNSRequest(domain, dns.TypeA, addEDNS)
 		binary.Write(c, binary.BigEndian, uint16(len(b)))
 		c.Write(b)

net/dns/manager_windows_test.go

@@ -502,7 +502,7 @@ func genRandomSubdomains(t *testing.T, n int) []dnsname.FQDN {
 	for len(domains) < cap(domains) {
 		l := r.Intn(19) + 1
 		b := make([]byte, l)
-		for i, _ := range b {
+		for i := range b {
 			b[i] = charset[r.Intn(len(charset))]
 		}
 		d := string(b) + ".example.com"

net/interfaces/interfaces_test.go

@@ -224,7 +224,7 @@ func TestStateString(t *testing.T) {
 					},
 				},
 				InterfaceIPs: map[string][]netip.Prefix{
-					"eth0": []netip.Prefix{
+					"eth0": {
 						netip.MustParsePrefix("10.0.0.2/8"),
 					},
 				},

net/portmapper/portmapper.go

@@ -961,7 +961,7 @@ var (
 	metricPMPSent = clientmetric.NewCounter("portmap_pmp_sent")
 
 	// metricPMPOK counts the number of times
-	// we received a succesful PMP response.
+	// we received a successful PMP response.
 	metricPMPOK = clientmetric.NewCounter("portmap_pmp_ok")
 
 	// metricPMPUnhandledOpcode counts the number of times

net/tsdial/tsdial.go

@@ -346,7 +346,7 @@ func (d *Dialer) dialPeerAPI(ctx context.Context, network, addr string) (net.Con
 //
 // The primary function of this is to work on macOS & iOS's in the
 // Network/System Extension so it can mark the dialer as staying
-// withing the network namespace/sandbox.
+// within the network namespace/sandbox.
 func (d *Dialer) getPeerDialer() *net.Dialer {
 	d.peerDialerOnce.Do(func() {
 		d.peerDialer = &net.Dialer{