crypto/tls: help linker remove code when only Client or Server is used

This saves 166 KiB for a tls.Dial hello world program (5382441 to
5212356 to bytes), by permitting the linker to remove TLS server code.

Change-Id: I16610b836bb0802b7d84995ff881d79ec03b6a84
Reviewed-on: https://go-review.googlesource.com/c/go/+/228111
Reviewed-by: Ian Lance Taylor <iant@golang.org>

tls.go

@@ -32,7 +32,12 @@ import (
 // The configuration config must be non-nil and must include
 // at least one certificate or else set GetCertificate.
 func Server(conn net.Conn, config *Config) *Conn {
-	return &Conn{conn: conn, config: config}
+	c := &Conn{
+		conn:   conn,
+		config: config,
+	}
+	c.handshakeFn = c.serverHandshake
+	return c
 }
 
 // Client returns a new TLS client side connection
@@ -40,7 +45,13 @@ func Server(conn net.Conn, config *Config) *Conn {
 // The config cannot be nil: users must set either ServerName or
 // InsecureSkipVerify in the config.
 func Client(conn net.Conn, config *Config) *Conn {
-	return &Conn{conn: conn, config: config, isClient: true}
+	c := &Conn{
+		conn:     conn,
+		config:   config,
+		isClient: true,
+	}
+	c.handshakeFn = c.clientHandshake
+	return c
 }
 
 // A listener implements a network listener (net.Listener) for TLS connections.