crypto/tls,crypto/x509: normalize RFC references

Use the format "RFC XXXX, Section X.X" (or "Appendix Y.X") as it fits
more properly in prose than a link, is more future-proof, and as there
are multiple ways to render an RFC. Capital "S" to follow the quoting
standard of RFCs themselves.

Applied the new goimports grouping to all files in those packages, too.

Change-Id: I01267bb3a3b02664f8f822e97b129075bb14d404
Reviewed-on: https://go-review.googlesource.com/c/141918
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

src/crypto/tls/handshake_client.go

@@ -845,7 +845,7 @@ func mutualProtocol(protos, preferenceProtos []string) (string, bool) {
 
 // hostnameInSNI converts name into an approriate hostname for SNI.
 // Literal IP addresses and absolute FQDNs are not permitted as SNI values.
-// See https://tools.ietf.org/html/rfc6066#section-3.
+// See RFC 6066, Section 3.
 func hostnameInSNI(name string) string {
 	host := name
 	if len(host) > 0 && host[0] == '[' && host[len(host)-1] == ']' {