More relevant VM backtrace

More relevant VM backtrace

* module/system/repl/repl.scm: Remove a useless print in the backtrace
  handler.

* module/system/vm/debug.scm (vm-backtrace): s/reverse!/reverse/

* module/system/vm/frame.scm (bootstrap-frame?): A heuristic to see if a
  frame is a bootstrap frame, the one allocated on the stack in
  vm_engine.c; need to have a better solution for this.
  (make-frame-chain): Don't include bootstrap frames, they add no
  information.
  (print-frame-chain-as-backtrace): Remove a pk.

* src/vm_engine.c (vm_run): Add a fixme about the bootstrap prograp.