More changes for "busy port" support

Since nanomsg does its own internal send buffering, use the driver queue
only to store data rejected from nanomsg due to an already-full send
buffer, and then use "busy port" to slow down senders under these
conditions. Add a new test to show that the port/socket eventually goes
into the busy state when enough data is sent and the receiver isn't
reading.
Check return value of nn_getsockopt