(usage): Say "blanks" instead of "whitespace",

(usage): Say "blanks" instead of "whitespace",
Similar fixes for many comments.
(TAB_DEFAULT): New constant, so that we can support NUL as
the field separator.
(tab): Now int, not char.  Initialize to TAB_DEFAULT.
(specify_sort_size): If multiple sizes are specified, use the largest.
(begfield, limfield): Support NUL tab char.
(set_ordering): Do not let -i override -d.
(main): Report an error if incompatible -o or -t options are given.
Report an error for "-t ''".  Allow "-t '\0'" to specify a NUL tab.