debugger - accessible luadebug frame

[tsafin]

up command could show you initial debuger frame:

There is a bug I do not know how to fix yet - due to the new mechanics with the new luadebug.lua::start entry-point we now are able to get deeper to the depths of a stack, to the level of start entry-point.

This frame should be hidden from user, and debugger should report bottom of a stack, not show actual frame inside of luadebug.lua.

Originally posted by @tsafin in #7929 (comment)