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/fakes/dest/sigterm_after_hello.rb

@@ -0,0 +1,19 @@
+#!/usr/bin/env ruby
+
+# Wait for a sender connection, send a correct hello, then sigterm the
+# sender.  We expect the sender to exit with status of 6, which is
+# enforced in the test.
+
+require 'flexnbd/fake_dest'
+include FlexNBD
+
+addr, port, pid = *ARGV
+server = FakeDest.new( addr, port )
+client = server.accept( "Timed out waiting for a connection" )
+client.write_hello
+
+Process.kill(15, pid.to_i)
+
+client.close
+server.close
+exit 0