Rockbox

Adds synchronous interthread message sending with a return value.

Features:
1) Can be #ifdef-ed away if a target doesn't require the functionality
2) Data is allocated and functionality enabled on a queue by queue basis with queue_enable_queue_send. Only one more pointer member in the event_queue structure
3) Uses scheduler blocking mechanism
4) Minimal overhead in checking if a message is sent or posted. Add one function call (queue_reply) to thread function with sendable queues.
5) Overflow safe
6) Interrupt safe (though interrupts must not call queue_send)
7) A sender knows the reply is for the senders particular post
8) queue_send automatically degrades to queue_post and returns NULL when sending is not enabled on the queue
9) Nets out at 164 bytes with only a single module using it. I expect savings in overall if used throughout when appropriate. callback_queue in playback.c should be able be removed and replaced with sending messages to the audio thread and receiving a return value since additional queues are currently used to implement message return values.

pcm_record.c modifications are included since that will be the first user of the new api.