all: rename variables with lowercase-l/uppercase-I

See http://go/no-ell

Signed-off-by: Alex Chan <alexc@tailscale.com>

Updates #cleanup

Change-Id: I8c976b51ce7a60f06315048b1920516129cc1d5d

net/dns/manager_windows_test.go

@@ -550,8 +550,8 @@ func genRandomSubdomains(t *testing.T, n int) []dnsname.FQDN {
 	const charset = "abcdefghijklmnopqrstuvwxyz"
 
 	for len(domains) < cap(domains) {
-		l := r.Intn(19) + 1
-		b := make([]byte, l)
+		ln := r.Intn(19) + 1
+		b := make([]byte, ln)
 		for i := range b {
 			b[i] = charset[r.Intn(len(charset))]
 		}