Previously, the push/pop API to switch between namespaces would always
switch both the net and mount namespace together.
There are situations, where we want to only switch one namespace.
For example, the function nmp_netns_bind_to_path() introduced next
only wants to switch the net namespace to get /proc/self/ns/net,
but must not switch the mount namespace as it bind-mounds in the
namespace of the caller.
GError is not used, the error branch would always result in NULL dereference.
Also, check for the result being zero for clarity -- it's the only allowed
success indication.
CID 75365 (#3 of 3): Explicit null dereferenced (FORWARD_NULL)
12. var_deref_op: Dereferencing null pointer error.
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
