Startup script fixes:

• Avoid race condition in startup. Use daemon(8) rather than "rabbitmqctl status" to safely retrieve the pid.

• Make sure that the Erlang cookie permissions are correct before starting the RabbitMQ node (using rabbitmqctl as root could create a read-only cookie).

• Make sure that the RabbitMQ application is actually running when rabbitmq_start is done. Remove the now useless "wait" command from extra_commands.