Free all possibly held mutexes in error handlers

Now that we have 3 mutexes lying around, it's important that we check
and free these if necessary if error() is called in any thread that can
hold them.  To do this, we now have flexthread.c, which defines a
flexthread_mutex struct.  This is a wrapper around a pthread_mutex_t and
a pthread_t.  The idea is that in the error handler, the thread can
check whether it holds the mutex and can free it if and only if it does.
This is important because pthread fast mutexes can be freed by *any*
thread, not just the thread which holds them.

Note: it is only ever safe for a thread to check if it holds the mutex
itself.  It is *never* safe to check if another thread holds a mutex
without first locking that mutex, which makes the whole operation rather
pointless.

tests/acceptance/flexnbd/fake_dest.rb

@@ -94,6 +94,31 @@ module FlexNBD
       end
 
 
+      def receive_mirror
+        write_hello()
+        loop do
+          req = read_request
+          case req[:type]
+          when 1
+            read_data( req[:len] )
+            write_reply( req[:handle] )
+          when 65536
+            write_reply( req[:handle] )
+            break
+          else
+            raise "Unexpected request: #{req.inspect}"
+          end
+        end
+
+        disc = read_request
+
+        if disc[:type] == 2
+          close
+        else
+          raise "Not a disconnect: #{req.inspect}"
+        end
+      end
+
     end # class Client