crypto/tls: load a chain of certificates from a file.

crypto/tls: load a chain of certificates from a file.

Many recently issued certificates are chained: there's one or more
intermediate certificates between the host certificate and the root CA
certificate. This change causes the code to load any number of
certificates from the certificate file. This matches the behaviour of
common webservers, and the output of OpenSSL's command line tools.

R=golang-dev, r2
CC=golang-dev
https://golang.org/cl/4119057