libct/init: unify init, fix its error logic

This commit does two things:

1. Consolidate StartInitialization calling logic into Init().
2. Fix init error handling logic.

The main issues at hand are:
- the "unable to convert _LIBCONTAINER_INITPIPE" error from
  StartInitialization is never shown;
- errors from WriteSync and WriteJSON are never shown;
- the StartInit calling code is triplicated;
- using panic is questionable.

Generally, our goals are:
 - if there's any error, do our best to show it;
 - but only show each error once;
 - simplify the code, unify init implementations.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/integration/init_test.go

@@ -1,9 +1,7 @@
 package integration
 
 import (
-	"fmt"
 	"os"
-	"runtime"
 	"testing"
 
 	"github.com/opencontainers/runc/libcontainer"
@@ -14,20 +12,9 @@ import (
 
 // Same as ../../init.go but for libcontainer/integration.
 func init() {
-	if len(os.Args) < 2 || os.Args[1] != "init" {
-		return
+	if len(os.Args) > 1 && os.Args[1] == "init" {
+		libcontainer.Init()
 	}
-	// This is the golang entry point for runc init, executed
-	// before TestMain() but after libcontainer/nsenter's nsexec().
-	runtime.GOMAXPROCS(1)
-	runtime.LockOSThread()
-	if err := libcontainer.StartInitialization(); err != nil {
-		// logrus is not initialized
-		fmt.Fprintln(os.Stderr, err)
-	}
-	// Normally, StartInitialization() never returns, meaning
-	// if we are here, it had failed.
-	os.Exit(1)
 }
 
 func TestMain(m *testing.M) {