Quotearg part 4: add tests, fix c-maybe colon quoting.

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>