,frame and related commands handle for-trap? appropriately

,frame and related commands handle for-trap? appropriately

* module/system/repl/debug.scm (print-frame): Add #:next-source? arg,
  for when print-frame should use frame-next-source instead of
  frame-source.
  (print-frames): Add #:for-trap? arg. If true, the 0th frame should be
  printed with frame-next-source.

* module/system/repl/command.scm (define-stack-command): Introduce
  for-trap? into the lexical env.
  (backtrace, up, down, frame): Update to do the right thing regarding
  #:for-trap?.