Added rangeset for file-offset allocation. File offsets can now be reused. :)

• Header file updated with usage instructions.
• set_max_grants ioctl removed.
• Refactored notify and event-channel notification code.

I'm sorry, I have very little idea about event channels, but from what I think, keeping refcounted channels is a better idea.
The event channel might be used to signal the foreign domain that we are unmapping the grant. Keeping a refcounted channel ensures that the notification always goes through, which won't be the case if we go with the other approach. The other domain may be waiting for a signal before deallocating the grant, and the lack of signal might lead to ghost grants accumulating, wasting precious memory (which in case of FreeBSD would be wired pages).

Hi bjk,

Added UNMAP_NOTIFY_SEND_EVENT code, factored out notify function, used a union to save space, and added usage instructions in the device header file.

Add notify function UNMAP_NOTIFY_CLEAR_BYTE.

Added Async cleanup using Taskqueue and RedBlack Tree for internal data structures.
Please review :)

The taskqueue(9) and tree(3) stuff has been tested very well as it is used in the rest of the system. There is no good reason not to use it from the start.

I've added comments to all parts of the code, and tried to conform with the style guidelines everywhere.
I'll add instructions on how to use the device in the header file in an hour.

Style Corrections