nspopen: proper `release()`
nspopen: proper `release()`

* do not release twice
* join the child process
* close all the queues

drzaeus77/pyroute2,nazarewk/pyroute2,little-dude/pyroute2,vodik/pyroute2,simudream/pyroute2,roolebo/pyroute2,little-dude/pyroute2,simudream/pyroute2,nazarewk/pyroute2,vodik/pyroute2,drzaeus77/pyroute2,roolebo/pyroute2,tomislacker/python-iproute2,mtiny/pyroute2,tomislacker/python-iproute2,nazarewk/pyroute2,mtiny/pyroute2,little-dude/pyroute2