Both `pr -0' and e.g., `pr -03' would evoke `column count too large'.

Both `pr -0' and e.g., `pr -03' would evoke `column count too large'.
`pr -0' should give a better diagnostic and `pr -03' should be
equivalent to `pr -3'.

(parse_column_count): Change return type to void.
Call error (EXIT_FAILURE, ... for an invalid string.
(main): Allocate space for column_count_string using malloc.
Accumulate all old-style column-count digits before converting.
When the number of columns is specified via both old-style,
(e.g., -3), and a long option (--columns=5), ensure that only
the last one specified takes effect.

Add tests for the above.