debug forward_list implementation

[Paolo Carlini]

... this is what I actually committed. As you can see, the debug.mode
forward_list already catched a bug... in the exception testing framework
itself ;) Seriously, the latter could be probably fixed also in a more
concise way, the below is correct, anyway.

Tested x86_64-linux, make check and make check-debug, committed.

Paolo.

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