Fix a multiple-sockets-in-one-process issue.

"pid" is not "process id" in netlink, but rather, "port id". If you bind to a
sockaddr == 0 then Linux automatically assigns your socket a port id - which
happens to be the same as the process ID for the first one concurrently open.

For the second and subsequent concurrently-open sockets, binding 0 (as most
users of this library will do) gets you back a random high-numbered port id.

This change preserves the existing use case (one port open in the process,
binding to 0) while fixing multiple-ports-open-in-the-same-process, socket-is-
passed-in-and-pid-is-not-specified, and specific-pid-is-requested-but-could-
not-bind-to-it.

We're probably still not thread-safe - the seq handling looks dodgy - but at
least now we can use multiple sockets in separate threads and have them all
work.

Using the same socket from multiple threads is a slightly niche use case, and
it's tempting to say "don't do this" instead...

lib/linux/netlink/nlsocket.rb

@@ -58,14 +58,16 @@ module Netlink
     #   :timeout => N (seconds, default to DEFAULT_TIMEOUT. Pass nil for no timeout)
     #   :junk_handler => lambda { ... } for unexpected packets
     def initialize(opt)
-      @socket ||= opt[:socket] || ::Socket.new(
+      @socket = opt[:socket] || ::Socket.new(
         Socket::AF_NETLINK,
         Socket::SOCK_DGRAM,
         opt[:protocol] || (raise "Missing :protocol")
       )
       @socket.bind(NLSocket.sockaddr(opt)) unless opt[:socket]
       @seq = opt[:seq] || Time.now.to_i
-      @pid = opt[:pid] || $$
+
+      @pid = @socket.getsockname.unpack(SOCKADDR_PACK)[2]
+
       @timeout = opt.has_key?(:timeout) ? opt[:timeout] : DEFAULT_TIMEOUT
       if opt.has_key?(:junk_handler)
         @junk_handler = opt[:junk_handler]
@@ -75,7 +77,7 @@ module Netlink
         }
       end
     end
-    
+
     # Close the Netlink socket
     def close
       @socket.close
@@ -85,7 +87,7 @@ module Netlink
     def next_seq
       @seq = (@seq + 1) & 0xffffffff
     end
-    
+
     # Add a header and send a single message over the socket.
     #   type:: the message type code
     #   msg:: the message to send (without header)
@@ -203,7 +205,7 @@ module Netlink
         end
       end
     end
-    
+
     # Receive one datagram from kernel. Validates the sender, and returns
     # the raw binary message. Raises an exception on timeout or if the
     # kernel closes the socket.