crypto/tls: allow the server to enforce its ciphersuite preferences.

Previously, Go TLS servers always took the client's preferences into
account when selecting a ciphersuite. This change adds the option of
using the server's preferences, which can be expressed by setting
tls.Config.CipherSuites.

This mirrors Apache's SSLHonorCipherOrder directive.

R=golang-dev, nightlyone, bradfitz, ality
CC=golang-dev
https://codereview.appspot.com/7163043