JOB_STACKFRAME_THUNK

JOB_STACKFRAME_THUNK stackframes in context look as follows:

  job_RunState.l   ->   size in bytes of below stackframe
                        JOB_STACKFRAME_THUNK
			stack slot in which to place thunk return value
                        saved logical-bottom-of-data-stack.
			saved actual_user
			saved acting_user
			saved privs
                        size in bytes of above stackframe

                        size in bytes of below stackframe
                        JOB_STACKFRAME_NORMAL   
                        local_variable_N
			...
			local_variable_1
  job_RunState.v   ->	local_variable_0
                        compiledFunction for this frame.
                        programCounter for this frame.
                        size in bytes of above stackframe

Rationale:

• A function may take any number of arguments, and any of them may prove to be a thunk requiring evaluation. We need to remember where in the stack to put the return value from the thunk when it completes execution.
• We need to create a logically empty data stack in order to prevent the thunk from messing with existing stack contents, which could be a security and/or privacy breach. Hence the need to save the old logical-bottom-of-data-stack for later restoration.
• We need to clear the privilege-conferring entries in the job state to innocuous values for similar security/privacy reasons, and must save them for later restoration before doing so. POSSIBLE BUG: It doesn't seem safe to clear current 'job' entry; If any special privileges are accessable via it, thunks may be a privacy/security hole.

A full getStackframe[ THUNK frame result looks like:

:owner             object
:kind              :thunk
:privileges	   fixnum
:actingUser       object
:actualUser       object
:stackBottom      fixnum
:return-slot       fixnum

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