crypto/tls: don't copy Mutex or Once values

This fixes some 40 warnings from go vet.

Fixes #16134.

Change-Id: Ib9fcba275fe692f027a2a07b581c8cf503b11087
Reviewed-on: https://go-review.googlesource.com/24287
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>

tls.go

@@ -135,9 +135,9 @@ func DialWithDialer(dialer *net.Dialer, network, addr string, config *Config) (*
 	// from the hostname we're connecting to.
 	if config.ServerName == "" {
 		// Make a copy to avoid polluting argument or default.
-		c := *config
+		c := config.clone()
 		c.ServerName = hostname
-		config = &c
+		config = c
 	}
 
 	conn := Client(rawConn, config)