(simple_cat, cat, main): Used char * rather than

(simple_cat, cat, main): Used char * rather than
unsigned char *, to avoid some technical violations of the C
standard, and to avoid the need for casts.  I guess the
unsigned char * was probably there for improved performance
with ancient compilers, but the code wasn't using the
unsignedness any more.
Reported by Nelson H. F. Beebe.

(cat): int -> size_t for full_write arg.  This doesn't
fix any bugs since the buffer size is small, but it makes
the code a bit clearer.