Add a namespace object interface for somewhat cleaner code when
creating and switching between network namespaces. All created
namespaces are now mounted in /var/run/netns to ensure they
have persistent inodes and paths that can be passed around
between plugin components without relying on the current namespace
being correct.
Also remove the thread-locking arguments from the ns package
per https://github.com/appc/cni/issues/183 by doing all the namespace
changes in a separate goroutine that locks/unlocks itself, instead of
the caller having to track OS thread locking.
Allow users to tune net network parameters such as somaxconn.
With this patch, users can add a new network configuration:
> {
> "name": "mytuning",
> "type": "tuning",
> "sysctl": {
> "net.core.somaxconn": "500"
> }
> }
The value /proc/sys/net/core/somaxconn will be set to 500 in the network
namespace but will remain unchanged on the host.
Only sysctl parameters that belong to the network subsystem can be
modified.
Related to: https://github.com/coreos/rkt/pull/2140
