Trailers are delivered to the application as headers via
CLIENTWRITE_TRAILER, but unlike regular response headers they skipped
the verify_header() checks, so a server could smuggle a nul byte (or
stray CR) into a header reaching CURLOPT_HEADERFUNCTION and
curl_easy_header().
Run each assembled trailer line through Curl_verify_header(), the same
validation used for normal headers.
Covered by the new test 2106.
Closes#21896
