Add a new NMPNetns class. This allows creation, deletion and
switching of network namespaces. The API only offers push/pop
operations to switch the namespace. This way the API enforces
the user to always restore the previous namespace.

A NMPlatform instance not only uses the netlink socket, but also
sysfs, udev, ethtool, mii. Still, a NMPlatform instance lives
entirely inside one namespace and is not spanning multiple namespaces.
To properly support network namespaces, the platform instance must
switch the namespace as necessary, transparent to the caller.
Udev is only supported in the main namespace.

For now, network namespaces are not actually used and are disabled
via the NM_PLATFORM_NETNS_SUPPORT argument.

https://bugzilla.gnome.org/show_bug.cgi?id=762408