Retry failed rebind attempts

When we receive a migration, if rebinding to the new listen address and
port fails for a reason which might be fixable, rather than killing the
server we retry once a second.  Also in this patch: non-overlapping log
messages and a fix for the client going away halfway through a sendfile
loop.

tests/acceptance/test_source_error_handling.rb

@@ -80,6 +80,7 @@ class TestSourceErrorHandling < Test::Unit::TestCase
 
 
   def test_post_entrust_disconnect_causes_retry
+    @env.nbd1.can_die(0)
     run_fake( "dest/close_after_entrust" )
   end