lib/rand: Various minor fixes (#6752)

crypto/rand output is cryptographically secure by the Go library
documentation's promise. That, rather than strength (= passes randomness
tests) is the property that Syncthing needs).

lib/tlsutil/tlsutil.go

@@ -102,7 +102,7 @@ func NewCertificate(certFile, keyFile, commonName string, lifetimeDays int) (tls
 	// NOTE: update checkExpiry() appropriately if you add or change attributes
 	// in here, especially DNSNames or IPAddresses.
 	template := x509.Certificate{
-		SerialNumber: new(big.Int).SetInt64(rand.Int63()),
+		SerialNumber: new(big.Int).SetUint64(rand.Uint64()),
 		Subject: pkix.Name{
 			CommonName: commonName,
 		},