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

src/pkg/crypto/rand/rand_unix.go

@@ -12,6 +12,7 @@ package rand
 import (
 	"bufio"
 	"crypto/aes"
+	"crypto/cipher"
 	"io"
 	"os"
 	"sync"
@@ -66,7 +67,7 @@ func newReader(entropy io.Reader) io.Reader {
 type reader struct {
 	mu                   sync.Mutex
 	budget               int // number of bytes that can be generated
-	cipher               *aes.Cipher
+	cipher               cipher.Block
 	entropy              io.Reader
 	time, seed, dst, key [aes.BlockSize]byte
 }