netlink: Use nl_pid 0 to play nice with others.

netlink: Use nl_pid 0 to play nice with others.

While the netlink field is named nl_pid, and typically
contains a process id, setting it to a pid value directly
prevents other modules from also using a netlink socket.
On the other hand, setting it to 0 allows multiple modules
to use a netlink socket by letting the kernel assign the
nl_pid a value.

This allows the verito tap code to interact nicely with
the router plugin's librtnl after, say, tap-inject has
been enabled.

Change-Id: I9771929f34d15497a5f7b8c5fd78dac28e31383b
Signed-off-by: Jon Loeliger <jdl@netgate.com>