An application can append a string to the right end of the existing query part
with the CURLU_APPENDQUERY flag.
Consider a handle that holds the URL https://example.com/?shoes=2. An
application can then add the string hat=1 to the query part like this:
rc = curl_url_set(urlp, CURLUPART_QUERY, "hat=1", CURLU_APPENDQUERY);
It even notices the lack of an ampersand (&) separator so it injects one
too, and the handle's full URL would then equal
https://example.com/?shoes=2&hat=1.
The appended string can of course also get URL encoded on add, and if asked,
the encoding skips the = character. For example, append candy=M&M to what
we already have, and URL encode it to deal with the ampersand in the data:
rc = curl_url_set(urlp, CURLUPART_QUERY, "candy=M&M",
CURLU_APPENDQUERY | CURLU_URLENCODE);
Now the URL looks like https://example.com/?shoes=2&hat=1&candy=M%26M.