CNI support for Windows

[TBBle]

These changes get the CNI integration working on Windows:

• Refactored NetNS handling to fully abstract the Linux codepath from common code, and implement a Windows codepath
• Disabled the 'loopback' CNI config on Windows, as it doesn't appear to exist, nor be needed.
• Disabled host network mode in Windows, because it doesn't appear to exist either, nor is it supported.
• Fixed a bug that would cause the --containerd-cni-binary-dir option to be ignored when reading the config provided by --containerd-cni-config-path.

Tested using a hack-bug-fixed version of the nat plugin from https://github.com/microsoft/windows-container-networking (see microsoft/windows-container-networking#57), with the following config from their examples.

{
    "cniVersion": "0.2.0",
    "name": "natNetwork",
    "type": "nat",
    "master": "Ethernet",
    "ipam": {
        "subnet": "192.168.100.0/24",
        "routes": [
            {
                "gateway": "192.168.100.1"
            }
        ]
    }
}

This is only used by the containerd worker on on Windows, which is not currently activated. I'm locally using a heavily-hacked-upon GenerateSpec to get an OCI spec that doesn't trigger LCOW support.

That's the last problem to solve, once the various underlying pieces are in-place and working.

Tested with the following Dockerfile:

FROM mcr.microsoft.com/windows/servercore:2004

LABEL Description="Python" Vendor="Python Software Foundation" Version="3.7.3"

RUN powershell.exe -Command \
    $ErrorActionPreference = 'Stop'; \
    [Net.ServicePointManager]::SecurityProtocol = [Net.SecurityProtocolType]::Tls12; \
    wget https://www.python.org/ftp/python/3.7.3/python-3.7.3.exe -OutFile c:\python-3.7.3.exe ; \
    Start-Process c:\python-3.7.3.exe -ArgumentList '/quiet InstallAllUsers=1 PrependPath=1' -Wait ; \
    Remove-Item c:\python-3.7.3.exe -Force

and confirmed that Python had been installed in the resulting image.

Contributes towards #616