Quotearg part 4: add tests, fix c-maybe colon quoting. * lib/quotearg.h: Improve documentation. * lib/quotearg.c (quotearg_buffer_restyled): Don't add extra escapes when adding outer quotes. When quoting trigraphs, use valid C notation. When quoting NUL, omit extra characters if next character is not digit. Alter prototype. (quotearg_buffer, quotearg_alloc_mem, quotearg_n_options): Adjust callers. * modules/quotearg-tests: New module. * tests/test-quotearg.c: New test. Signed-off-by: Eric Blake <ebb9@byu.net>