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/environment.rb

@@ -121,11 +121,12 @@ class Environment
 
   def run_fake( name, addr, port, sock=nil )
     fakedir = File.join( File.dirname( __FILE__ ), "fakes" )
-    fake = Dir[File.join( fakedir, name ) + "*"].sort.find { |fn|
+    fakeglob = File.join( fakedir, name ) + "*"
+    fake = Dir[fakeglob].sort.find { |fn|
       File.executable?( fn )
     }
 
-    raise "no fake executable" unless fake
+    raise "no fake executable at #{fakeglob}" unless fake
     raise "no addr" unless addr
     raise "no port" unless port