Add support for loading X.509 key pairs that consist of a certificate
with an EC public key and its corresponding EC private key.
R=agl
CC=golang-dev
https://golang.org/cl/6776043
X509KeyPair wasn't really supposed to allow the certificate and
key to be in the same file, but it did work if you put the key
first. Since some HTTPS servers support loading keys and certs
like this, this change makes it work in either order.
Fixes#3986.
R=golang-dev, dave, rsc
CC=golang-dev
https://golang.org/cl/6499103