maint: enable -Wsuggest-attribute=format

maint: enable -Wsuggest-attribute=format

* configure.ac (WERROR_CFLAGS): Enable -Wsuggest-attribute=format
for lib/ and src/.
* src/copy.c (copy_attr_error, copy_attr_allerror):
Add ATTRIBUTE_FORMAT.
(copy_attr): Ignore -Wsuggest-attribute=format in the
small section of code that needs it ignored.
* src/test.c (test_syntax_error): Mark with ATTRIBUTE_FORMAT.
(binary_operator): Omit unnecessary NULL args, pacifying
-Wsuggest-attribute=format.