Avoid doubling \ in common case of "c-maybe" quoting style.

Avoid doubling \ in common case of "c-maybe" quoting style.

* lib/quotearg.c (quotearg_buffer_restyled): Don't escape \ when
eliding outer quotes.
* lib/quotearg.h: Document this.
* tests/test-quotearg.c (result_strings, inputs, results_g)
(flag_results, locale_results): Test it by adding a new string to
each test group.
(compare_strings): Test new string.

Signed-off-by: Eric Blake <ebb9@byu.net>