Add support for the DTrace struct proc and struct thread extended data via ctor and dtor event handlers.
Add support for the DTrace struct proc and struct thread extended
data via ctor and dtor event handlers.

The size of the extra data is allocated opaquely and this file
contains a function which the dtrace module can call to check
that the kernel supports at least the amount of data that it needs.

This file is optionally compiled into nthe kernel if the KDTRACE_HOOKS
kernel option is defined.

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