sys.os_name_human and sys.os_version_major policy variables defined in C code

For the MP UI, we want a OS inventory attribute which is as short and readable / familiar as possible, regardless of what the OS vendor prefers as their OS / pretty name.

We made that change here:

https://tracker.mender.io/browse/ENT-6536

https://github.com/cfengine/masterfiles/pull/1868

However, the policy becomes messy. If we had these 2 variables it would be a lot easier:

vars:
  "description"
    string => "$(sys.os_name_human) $(sys.os_version_major)",
    meta => { "inventory", "attribute_name=OS" };

sys.os_name_human

• RHEL for Red Hat Enterprise Linux
• CentOS for CentOS Linux
• SUSE for SUSE Linux Enterprise Server
• Ubuntu for Ubuntu Linux
• Debian for Debian GNU/Linux
• Windows for both Windows Home, Professional, Server, etc.
• macOS for apple's operating system
• HP-UX for Hewlett Packard Unix

sys.os_version_major

• 18 for Ubuntu 18.04
• 7 for CentOS Linux 7.6
• 2016 for WIndows Server 2016

This ticket is limited to only the 2 variables we need because: limit scope, hard to know where to stop, and hard to know exactly how to put OS versions into major-minor-patch-build etc. (Where does SP2 go?).

sys.os_version_minor and sys.os_version_patch etc. could be implemented in the future, once we make a decision on exactly what those should be.