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