Remove listen mode

Changing behaviour so that instead of rebinding after a successful
migration and continuing as an ordinary server, we simply quit with a
0 exit code and let our caller restart us as a server if they want to.
This means that everything in listen.c, listen.h, and anything making
reference to a rebind address is unneeded.

tests/acceptance/fakes/source/close_after_write.rb

@@ -12,10 +12,11 @@ addr, port, srv_pid = *ARGV
 
 client = FakeSource.new( addr, port, "Timed out connecting" )
 client.read_hello
-Process.kill( "STOP", srv_pid.to_i )
+
+system "kill -STOP #{srv_pid}"
 client.write_write_request( 0, 8 )
 client.close
-Process.kill( "CONT", srv_pid.to_i )
+system "kill -CONT #{srv_pid}"
 
 # This sleep ensures that we don't return control to the test runner
 # too soon, giving the flexnbd process time to fall over if it's going