Fix the usage of WithNetNS* functions

[brb]

As it's been mentioned in #2388 , the WithNetNS* family of functions and their usage are not safe. To address the issues, the work function has to be executed in a separate process which is not associated with the Go runtime process which created it.

Currently, WithNetNS* is used by the following functions:

• common/utils.go:FindNetDevs
• common/utils.go:GetWeaveNetDevs
• net/veth.go:interfaceExistsInNamespace
• net/veth.go:AttachContainer
• net/veth.go:DetachContainer
• plugin/net/cni.go:CmdAdd
• plugin/net/cni.go:CmdDel

In some cases, shelling out is not necessary - it's enough to move a netns dependant code to the end of execution and to ensure that the code does not create any go-routine which require a particular netns.

[brb]

plugin/net/cni.go:CmdAdd: it does not require any changes, because the WithNetNS* call happens at the very end of execution and the CNI plugin stops a process once it has finished processing the request.

[brb]

As a subtask: add a shim for starting a process or do the init hack to make sure that all threads of the runtime is in a given netns. Otherwise, we cannot create any go-routine inside a former work() function.