platform: support switching partial namespaces

platform: support switching partial namespaces

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.