host,pkg/rpcplus: Use os.File instead of ClosingFD

This prevents the c.stdin file from being double closed by the
finalizer in containerinit by closing it explicitly in fdrpc.

Closes #402
Closes #384

Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>
Merge pull request #412 from flynn/fix-402

host,pkg/rpcplus: Use os.File instead of ClosingFD