fix the order of setns()

You cannot attach to namespaces in random order.
If you enter the mnt namespace before pid your procfs will go nuts.
Note that readdir() doesn't guarantee any ordering.
Here's a patch which you or @maebashi can apply:

```diff
From 0de149b5949173230c00c201629fc62990c95fe1 Mon Sep 17 00:00:00 2001
From: Aleksa Sarai <cyphar@cyphar.com>
Date: Thu, 24 Jul 2014 17:34:00 +1000
Subject: [PATCH] namespaces: nsenter: ignore nonexistent namespaces

If a particular kernel doesn't have namespace directories that
libcontainer tries to attain during nsenter(), ignore the error (this is
consistent with the previous implementation of nsenter()).

Docker-DCO-1.1-Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)
---
 namespaces/nsenter.go | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/namespaces/nsenter.go b/namespaces/nsenter.go
index ddc2541..7784b9f 100644
--- a/namespaces/nsenter.go
+++ b/namespaces/nsenter.go
@@ -135,6 +135,10 @@ void nsenter() {
                snprintf(buf, PATH_MAX - 1, "%s%s", ns_dir, namespaces[i]);
                int fd = open(buf, O_RDONLY);
                if (fd == -1) {
+                       // Ignore nonexistent namespaces.
+                       if (errno == ENOENT)
+                               continue;
+
                        fprintf(stderr, "nsenter: Failed to open ns file \"%s\" for ns \"%s\" with error: \"%s\"\n", buf, namespaces[i], strerror(errno));
                        exit(1);
                }
-- 
2.0.2

```

Unfortunately, I cannot seem to build libcontainer at the moment, so if you wouldn't mind checking that the build works with this patch applied.