Quotearg part 2: add flag that can control NUL elision.
* lib/quotearg.h (set_quoting_flags): New prototype.
* lib/quotearg.c (struct quoting_options): Add flag field.
(set_quoting_flags): New function.
(quotearg_buffer_restyled): Add flags parameter.
(quotearg_alloc_mem): Set the flag if length cannot be returned.
(quotearg_n_options): Set the flag, since length cannot be
returned.
(quoting_options_from_style): Default flags correctly.