This changeset implements client certificate support in crypto/tls
for both handshake_server.go and handshake_client.go

The updated server implementation sends an empty CertificateAuthorities
field in the CertificateRequest, thus allowing clients to send any
certificates they wish. Likewise, the client code will only respond
with its certificate when the server requests a certificate with this
field empty.

R=agl, rsc, agl1
CC=golang-dev
https://golang.org/cl/1975042