quotearg: implement custom_quoting_style

quotearg: implement custom_quoting_style

* lib/quotearg.c: (struct quoting_options): Add left_quote and
right_quote fields.
(set_custom_quoting): New public function.
(quotearg_buffer_restyled): Add left_quote and right_quote
arguments, handle them very much like locale quoting, and update
all uses.
(quotearg_n_custom): New public function.
(quotearg_n_custom_mem): New public function.
(quotearg_custom): New public function.
(quotearg_custom_mem): New public function.
* lib/quotearg.h: Prototype and document new public functions.
(enum quoting_style): For escape_quoting_style and
clocale_quoting_style, comment that QA_SPLIT_TRIGRAPHS is
ignored even though they're otherwise like c_quoting_style.
Add custom_quoting_style member and document with comparison to
clocale_quoting_style.
* tests/test-quotearg.c (custom_quotes): New array.
(custom_results): New array.
(main): Extend to test custom quoting.