Switch language name 'zh' to 'zh-cn'

This is the first step to rename 'zh' to 'zh-cn'. There are several reasons why we rename the language name.

- The upstream docsy theme changed the language name, leading to many warnings during site build;
  The side-effect is that the i18n strings are no longer working.
- We believe renaming the language is the right thing to do, because this move can make room for other variants of Chinese language, such as 'zh-tw', 'zh-sg' etc.

There would be several follow-ups to this PR, such as fixing the intra-site links, adding redirects etc.

We will lock up changes to zh/zh-cn pages for the moment, until this one gets in.

This PR is based on commit cdad0a7342.

content/zh-cn/docs/reference/glossary/sysctl.md

@@ -0,0 +1,51 @@
+---
+title: sysctl
+id: sysctl
+date: 2019-02-12
+full_link: /zh/docs/tasks/administer-cluster/sysctl-cluster/
+short_description: >
+  用于获取和设置 Unix 内核参数的接口
+
+aka:
+tags:
+- 工具
+---
+
+<!--
+---
+title: sysctl
+id: sysctl
+date: 2019-02-12
+full_link: /docs/tasks/administer-cluster/sysctl-cluster/
+short_description: >
+  An interface for getting and setting Unix kernel parameters
+
+aka:
+tags:
+- tool
+---
+-->
+
+<!--
+ `sysctl` is a semi-standardized interface for reading or changing the
+ attributes of the running Unix kernel.
+-->
+
+  `sysctl` 是一个半标准化的接口，用于读取或更改正在运行的 Unix 内核的属性。
+
+<!--more-->
+
+<!--
+On Unix-like systems, `sysctl` is both the name of the tool that administrators
+use to view and modify these settings, and also the system call that the tool
+uses.
+-->
+
+在类 Unix 系统上， `sysctl` 既是管理员用于查看和修改这些设置的工具的名称，也是该工具所调用的系统调用的名称。
+
+<!--
+{{< glossary_tooltip text="Container" term_id="container" >}} runtimes and
+network plugins may rely on `sysctl` values being set a certain way.
+-->
+
+{{< glossary_tooltip text="容器" term_id="container" >}}运行时和网络插件可能对 `sysctl` 的取值有一定的要求。