new plugin: tuning

Allow users to tune net network parameters such as somaxconn.

With this patch, users can add a new network configuration:

> {
>   "name": "mytuning",
>   "type": "tuning",
>   "sysctl": {
>           "net.core.somaxconn": "500"
>   }
> }

The value /proc/sys/net/core/somaxconn will be set to 500 in the network
namespace but will remain unchanged on the host.

Only sysctl parameters that belong to the network subsystem can be
modified.

Related to: https://github.com/coreos/rkt/pull/2140

diff --git a/tuning.md b/tuning.md
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+# tuning plugin
+
+## Overview
+
+This plugin can change some system controls (sysctls) in the network namespace.
+It does not create any network interfaces and therefore does not bring connectivity by itself.
+It is only useful when used in addition to other plugins.
+
+## Operation
+The following network configuration file
+```
+{
+  "name": "mytuning",
+  "type": "tuning",
+  "sysctl": {
+          "net.core.somaxconn": "500"
+  }
+}
+```
+will set /proc/sys/net/core/somaxconn to 500.
+Other sysctls can be modified as long as they belong to the network namespace (`/proc/sys/net/*`).
+
+A successful result would simply be:
+```
+{
+  "cniVersion": "0.1.0"
+}
+```
+
+## Network sysctls documentation
+
+Some network sysctls are documented in the Linux sources:
+
+- [Documentation/sysctl/net.txt](https://www.kernel.org/doc/Documentation/sysctl/net.txt)
+- [Documentation/networking/ip-sysctl.txt](https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt)
+- [Documentation/networking/](https://www.kernel.org/doc/Documentation/networking/)