Include mbswidth.h.

Include mbswidth.h.
(standard_header, header, test_suite): Remove.
(date_format, date_text, file_text, header_width_available): New vars.
(long_options, main, init_header, usage):
Add new -D or --date-format option.
(CHARS_FOR_DATE_AND_PAGE, T_BUF_FMT, T_BUF_SIZE, NO_DATE): Remove.
(init_header): Allow arbitrary width for date format.  Change
"Page %5d" to "Page %d", since the code no longer assumes fixed width.
Do not assume that localtime succeeds.
(init_header, print_header, usage): Do not truncate headers.
(init_header, print_header): Defer width calculations until
page is printed, since "Page 100000" is wider than "Page 1".
Count columns, not bytes, in page headers.
Custom headers take up only the center, not the whole header.
(print_header): Use printf rather than fprintf(stdout).