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/README.md

@@ -23,22 +23,9 @@ function as the entry of "bootstrap".
 In addition to the go init function the early stage bootstrap is handled by importing
 [nsenter](https://github.com/opencontainers/runc/blob/master/libcontainer/nsenter/README.md).
 
-```go
-import (
-	_ "github.com/opencontainers/runc/libcontainer/nsenter"
-)
-
-func init() {
-	if len(os.Args) > 1 && os.Args[1] == "init" {
-		runtime.GOMAXPROCS(1)
-		runtime.LockOSThread()
-		if err := libcontainer.StartInitialization(); err != nil {
-			logrus.Fatal(err)
-		}
-		panic("--this line should have never been executed, congratulations--")
-	}
-}
-```
+For details on how runc implements such "init", see
+[init.go](https://github.com/opencontainers/runc/blob/master/init.go)
+and [libcontainer/init_linux.go](https://github.com/opencontainers/runc/blob/master/libcontainer/init_linux.go).
 
 Then to create a container you first have to create a configuration
 struct describing how the container is to be created. A sample would look similar to this: