pack_unpack: List unrecognized keys in strict unpacking

Otherwise figuring out what's wrong with your JSON can be tricky,
especially if you're using a single fmt string to validate a large,
complicated schema.

The comma delimiting will make separating keys that contain commas
difficult.  For example:

  {"foo, bar": true, "baz": false}

will generate errors like:

  2 object item(s) left unpacked: foo, bar, baz

but that seems like a small enough corner case to not be worth much
worrying.

I wanted to find a way to handle this without have_unrecognized_keys,
but the strbuffer tooling makes it look like I shouldn't be reaching
in to do things like:

  strbuffer_t unrecognized_keys;
  unrecognized_keys.value = NULL;

and then using 'unrecognized_keys.value == NULL' in place of
have_unrecognized_keys.