crypto/tls: support CBC ciphers

crypto/tls: support CBC ciphers

This is largely based on ality's CL 2747042.

crypto/rc4: API break in order to conform to crypto/cipher's
Stream interface

cipher/cipher: promote to the default build

Since CBC differs between TLS 1.0 and 1.1, we downgrade and
support only 1.0 at the current time. 1.0 is what most of the
world uses.

Given this CL, it would be trival to add support for AES 256,
SHA 256 etc, but I haven't in order to keep the change smaller.

R=rsc
CC=ality, golang-dev
https://golang.org/cl/3659041