use epoll to manage console i/o in linux

this adds a `platform` interface for shim service to manage platform-specific
behaviors such as I/O (which uses epoll in linux to work around bugs with applications
that closes all consoles i.e. https://github.com/opencontainers/runc/pull/1434
and https://github.com/moby/moby/issues/27202)

Its expected that we only have 1 epollfd per containerd_shim to manage all processes.
Since all the work are done outside of the container runtime, upgrading of runc
is not required and should be done separately.

Signed-off-by: Daniel Dao <dqminh89@gmail.com>

linux/shim/service_unix.go

@@ -0,0 +1,58 @@
+// +build !windows,!linux
+
+package shim
+
+import (
+	"io"
+	"sync"
+	"syscall"
+
+	"github.com/containerd/console"
+	"github.com/containerd/fifo"
+	"golang.org/x/net/context"
+)
+
+type unixPlatform struct {
+}
+
+func (p *unixPlatform) copyConsole(ctx context.Context, console console.Console, stdin, stdout, stderr string, wg, cwg *sync.WaitGroup) (console.Console, error) {
+	if stdin != "" {
+		in, err := fifo.OpenFifo(ctx, stdin, syscall.O_RDONLY, 0)
+		if err != nil {
+			return nil, err
+		}
+		cwg.Add(1)
+		go func() {
+			cwg.Done()
+			io.Copy(console, in)
+		}()
+	}
+	outw, err := fifo.OpenFifo(ctx, stdout, syscall.O_WRONLY, 0)
+	if err != nil {
+		return nil, err
+	}
+	outr, err := fifo.OpenFifo(ctx, stdout, syscall.O_RDONLY, 0)
+	if err != nil {
+		return nil, err
+	}
+	wg.Add(1)
+	cwg.Add(1)
+	go func() {
+		cwg.Done()
+		io.Copy(outw, console)
+		console.Close()
+		outr.Close()
+		outw.Close()
+		wg.Done()
+	}()
+	return console, nil
+}
+
+func (p *unixPlatform) shutdownConsole(ctx context.Context, cons console.Console) error {
+	return nil
+}
+
+func (s *Service) initPlatform() error {
+	s.platform = &unixPlatform{}
+	return nil
+}