crypto/...: more fixes for bug 2841

1) Remove the Reset() member in crypto/aes and crypto/des (and
   document the change).
2) Turn several empty error structures into vars. Any remaining error
   structures are either non-empty, or will probably become so in the
   future.
3) Implement SetWriteDeadline for TLS sockets. At the moment, the TLS
   status cannot be reused after a Write error, which is probably fine
   for most uses.
4) Make crypto/aes and crypto/des return a cipher.Block.

R=rsc, r
CC=golang-dev
https://golang.org/cl/5625045

conn.go

@@ -87,9 +87,9 @@ func (c *Conn) RemoteAddr() net.Addr {
 	return c.conn.RemoteAddr()
 }
 
-// SetDeadline sets the read deadline associated with the connection.
-// There is no write deadline.
-// A zero value for t means Read will not time out.
+// SetDeadline sets the read and write deadlines associated with the connection.
+// A zero value for t means Read and Write will not time out.
+// After a Write has timed out, the TLS state is corrupt and all future writes will return the same error.
 func (c *Conn) SetDeadline(t time.Time) error {
 	return c.conn.SetDeadline(t)
 }
@@ -100,10 +100,11 @@ func (c *Conn) SetReadDeadline(t time.Time) error {
 	return c.conn.SetReadDeadline(t)
 }
 
-// SetWriteDeadline exists to satisfy the net.Conn interface
-// but is not implemented by TLS.  It always returns an error.
+// SetWriteDeadline sets the write deadline on the underlying conneciton.
+// A zero value for t means Write will not time out.
+// After a Write has timed out, the TLS state is corrupt and all future writes will return the same error.
 func (c *Conn) SetWriteDeadline(t time.Time) error {
-	return errors.New("TLS does not support SetWriteDeadline")
+	return c.conn.SetWriteDeadline(t)
 }
 
 // A halfConn represents one direction of the record layer
@@ -726,9 +727,13 @@ func (c *Conn) readHandshake() (interface{}, error) {
 }
 
 // Write writes data to the connection.
-func (c *Conn) Write(b []byte) (n int, err error) {
-	if err = c.Handshake(); err != nil {
-		return
+func (c *Conn) Write(b []byte) (int, error) {
+	if c.err != nil {
+		return 0, c.err
+	}
+
+	if c.err = c.Handshake(); c.err != nil {
+		return 0, c.err
 	}
 
 	c.out.Lock()
@@ -737,10 +742,10 @@ func (c *Conn) Write(b []byte) (n int, err error) {
 	if !c.handshakeComplete {
 		return 0, alertInternalError
 	}
-	if c.err != nil {
-		return 0, c.err
-	}
-	return c.writeRecord(recordTypeApplicationData, b)
+
+	var n int
+	n, c.err = c.writeRecord(recordTypeApplicationData, b)
+	return n, c.err
 }
 
 // Read can be made to time out and return a net.Error with Timeout() == true