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/test_source_error_handling.rb

@@ -19,12 +19,24 @@ class TestSourceErrorHandling < Test::Unit::TestCase
   end
 
 
+  def expect_term_during_migration
+    @env.nbd1.can_die(6,9)
+  end
+
+
   def test_failure_to_connect_reported_in_mirror_cmd_response
     stdout, stderr = @env.mirror12_unchecked
+    expect_term_during_migration
     assert_match( /failed to connect/, stderr )
   end
 
 
+  def test_sigterm_after_hello_quits_with_status_of_1
+    expect_term_during_migration
+    run_fake( "dest/sigterm_after_hello" )
+  end
+
+
   def test_destination_hangs_after_connect_reports_error_at_source
     run_fake( "dest/hang_after_connect",
               :err => /Remote server failed to respond/ )
@@ -36,6 +48,7 @@ class TestSourceErrorHandling < Test::Unit::TestCase
               :err => /Mirror was rejected/ )
   end
 
+
   def test_wrong_size_causes_disconnect
     run_fake( "dest/hello_wrong_size",
               :err => /Remote size does not match local size/ )
@@ -43,38 +56,45 @@ class TestSourceErrorHandling < Test::Unit::TestCase
 
 
   def test_wrong_magic_causes_disconnect
+    expect_term_during_migration
     run_fake( "dest/hello_wrong_magic",
               :err => /Mirror was rejected/ )
   end
 
 
   def test_disconnect_after_hello_causes_retry
+    expect_term_during_migration
     run_fake( "dest/close_after_hello",
               :out => /Mirror started/ )
   end
 
 
   def test_write_times_out_causes_retry
+    expect_term_during_migration
     run_fake( "dest/hang_after_write" )
   end
 
 
   def test_rejected_write_causes_retry
+    expect_term_during_migration
     run_fake( "dest/error_on_write" )
   end
 
 
   def test_disconnect_before_write_reply_causes_retry
+    expect_term_during_migration
     run_fake( "dest/close_after_write" )
   end
 
 
   def test_bad_write_reply_causes_retry
+    expect_term_during_migration
     run_fake( "dest/write_wrong_magic" )
   end
 
 
   def test_pre_entrust_disconnect_causes_retry
+    expect_term_during_migration
     run_fake( "dest/close_after_writes" )
   end