x/crypto/ssh/agent: support smartcard keys

[aaazalea]

Please answer these questions before submitting your issue. Thanks!

1. What version of Go are you using (go version)?

go version go1.6.2 darwin/amd64

1. What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH=""
GORACE=""
GOROOT="/usr/local/Cellar/go/1.6.2/libexec"
GOTOOLDIR="/usr/local/Cellar/go/1.6.2/libexec/pkg/tool/darwin_amd64"
GO15VENDOREXPERIMENT="1"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fno-common"
CXX="clang++"
CGO_ENABLED="1"

1. What did you do?

This doesn't particularly apply, it's that I want to be able to use the client to add and remove smartcard keys. The agent API for this is pretty simple, but the Go implementation ignores them.

1. What did you expect to see?

an implementation

1. What did you see instead?

an unused variable for the protocol number and that's it.

[aaazalea]

https://golang.org/doc/contribute.html suggests that I should claim an issue before working on it, but I can't assign myself this, and I don't know how claiming would otherwise work.

[bradfitz]

I don't know of anybody else working on it, so consider it yours.

/cc @hanwen @agl