New Argument for HeapTrc

[skiy1337lazarus]

I thought it would be nice to have a megabytes parameter for HeapTrc, so that memory less than this amount will NOT be freed -- but rather FillChar on it with #255.

This is useful for debugging dangling pointers (objects used after destruction.)

My idea:
http://forum.lazarus.freepascal.org/index.php/topic,27880.0.html