## nsenter

The `nsenter` package registers a special init constructor that is called before the Go runtime has 
a chance to boot.  This provides us the ability to `setns` on existing namespaces and avoid the issues
that the Go runtime has with multiple threads.  This constructor is only called if this package is 
registered, imported, in your go application and the argv 0 is `nsenter`.

## nsenter

The `nsenter` package registers a special init constructor that is called before 
the Go runtime has a chance to boot.  This provides us the ability to `setns` on 
existing namespaces and avoid the issues that the Go runtime has with multiple 
threads.  This constructor will be called if this package is registered, 
imported, in your go application.

The `nsenter` package will `import "C"` and it uses [cgo](https://golang.org/cmd/cgo/)
package. In cgo, if the import of "C" is immediately preceded by a comment, that comment, 
called the preamble, is used as a header when compiling the C parts of the package.
So every time we  import package `nsenter`, the C code function `nsexec()` would be 
called. And package `nsenter` is now only imported in Docker execdriver, so every time 
before we call `execdriver.Exec()`, that C code would run.

`nsexec()` will first check the environment variable `_LIBCONTAINER_INITPID` 
which will give the process of the container that should be joined. Namespaces fd will 
be found from `/proc/[pid]/ns` and set by `setns` syscall.

And then get the pipe number from `_LIBCONTAINER_INITPIPE`, error message could
be transfered through it. If tty is added, `_LIBCONTAINER_CONSOLE_PATH` will 
have value and start a console for output.

Finally, `nsexec()` will clone a child process , exit the parent process and let 
the Go runtime take over.

Add more explanation for nsenter
Add more explanation for nsenter

Now the README in nsenter is a little confused
and hard for new commings to understand.

Signed-off-by: Sun Jianbo <d62b3f91fa43fa1a4d86143f0646a68a9fdeaa6e@zju.edu.cn>
