New version of tac.c

[Tito]

Hi,
this new version of tac.c handles embedded nulls in a correct way.
I tested it in comparison with the real tac on most of the files 
of my hard disk with the attached script and it seems to work.
Before posting a patch I would like the list members to take
a look at this new approach for hints, improvements and critics.
For the same reason I have not checked the size increase yet.

Ciao,
Tito
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