new-api: execute a process inside an existing container

A new constructor function (like nsenter) is added in this patch.  This
function gets arguments from environment variables and its behaviour doesn't
depend on a command line arguments.

A program which calls factory.StartInitialization() must import the nsenter
package. It looks ugly, but I don't know another way how to enter into CT from
a go code.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
new-api: set Cloneflags in namespace.Exec()

This place looks more suitable.

Signed-off-by: Andrey Vagin <avagin@openvz.org>