netlink: iothread improvements
netlink: iothread improvements

* make iothread possible to handle several netlink sockets
* assign each input socket it's own marshal

prepare iothread for netlink routing
nazarewk/pyroute2,nazarewk/pyroute2,vodik/pyroute2,drzaeus77/pyroute2,tomislacker/python-iproute2,nazarewk/pyroute2,mtiny/pyroute2,little-dude/pyroute2,tomislacker/python-iproute2,simudream/pyroute2,little-dude/pyroute2,mtiny/pyroute2,roolebo/pyroute2,drzaeus77/pyroute2,vodik/pyroute2,roolebo/pyroute2,simudream/pyroute2,little-dude/pyroute2