arcwatch implementation

Initial implementation. Adds a new zfs-ioctl ARC_INFO to dump all headers
currently in arc.
Adds a utility 'arcwatch' to dump the arc and to trace a command to see
which buff

arcwatch implementation

Initial implementation. Adds a new zfs-ioctl ARC_INFO to dump all headers
currently in arc.
Adds a utility 'arcwatch' to dump the arc and to trace a command to see
which buffers it needs.

(cherry picked from commit a8c92b49f4e8de856d9e4775e15e4fa90a687664)

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