Quit with an error status on SIGTERM during migration

This prevents the supervisor from thinking that the migration completed
successfully.

In order to do this, I've introduced a new lock around the start (and
finish) of the migration so that we avoid a race between the signal
handler in the server_accept loop and the control thread mirror startup.
Without that, we'd risk successfully starting a migration after the
SIGTERM handler fired, which would be Bad.

tests/acceptance/flexnbd.rb

@@ -136,11 +136,11 @@ class ValgrindKillingExecutor
 
 
   def call( err )
-    Process.kill( "KILL", @pid )
     $stderr.puts "*"*72
     $stderr.puts "* Valgrind error spotted:"
     $stderr.puts err.to_s.split("\n").map{|s| "  #{s}"}
     $stderr.puts "*"*72
+    Process.kill( "KILL", @pid )
     exit(1)
   end
 
@@ -323,7 +323,8 @@ module FlexNBD
 
 
     def serve( file, *acl)
-      run_serve_cmd( serve_cmd( file, acl ) )
+      cmd = serve_cmd( file, acl )
+      run_serve_cmd( cmd )
     end
 
     def listen(file, *acl)