(add_tab_stop): Renamed from add_tabstop. All uses changed.

(add_tab_stop): Renamed from add_tabstop.  All uses changed.
(parse_tab_stop): Renamed from parse_tabstop.  All uses changed.
(validate_tab_stop): Renamed from validate_tabstop.  All uses changed.
(next_file, main): Check fclose against 0, not EOF.
(unexpand): Remove unnecessary casts.
Add another loop nesting level, for lines, so that per-line variables
are initialized cleanly.
Revamp tab checking.  Check for write error immediately, rather
than just once at the end of the program.
(TAB_STOP_SENTINEL): Remove.
(tab_size): Now size_t, not uintmax_t, since we need to store
the sequences of blanks.
(max_column_width): New var.
(usage): Say "blank" where POSIX requires this.
(add_tab_stop): Calculate maximum column width.
(unexpand): Store the pending blanks, instead of merely counting them.
Follow POSIX's rules about -a requiring two blanks before a tab stop.
Get rid of internal label and goto.