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>

init.go

@@ -2,7 +2,6 @@ package main
 
 import (
 	"os"
-	"runtime"
 
 	"github.com/opencontainers/runc/libcontainer"
 	_ "github.com/opencontainers/runc/libcontainer/nsenter"
@@ -12,14 +11,6 @@ func init() {
 	if len(os.Args) > 1 && os.Args[1] == "init" {
 		// This is the golang entry point for runc init, executed
 		// before main() but after libcontainer/nsenter's nsexec().
-		runtime.GOMAXPROCS(1)
-		runtime.LockOSThread()
-
-		if err := libcontainer.StartInitialization(); err != nil {
-			// as the error is sent back to the parent there is no need to log
-			// or write it to stderr because the parent process will handle this
-			os.Exit(1)
-		}
-		panic("libcontainer: container init failed to exec")
+		libcontainer.Init()
 	}
 }