New version of tac.c

[Tito]

Hi Denis,
couldn't we change this 0x7f 

if (!(i & 0x7f))
    /* Grow on every 128th char */
    line = xrealloc(line, i + 0x7f + sizeof(int) + 1);

into a define

#define WHATEVER                          0x7f

so that it could be changed in one place
and to improve readability for newbies like me. :-)

Just an idea.

Ciao,
Tito