configure: ac_cv_ -> curl_cv_ for all cached vars

This was automated by:

sed -b -i -f <(ack -A1 AC_CACHE_CHECK | \
               ack -o 'ac_cv_.*?\b' | \
               sort -u | xargs -n1 bash -c \
                    'echo "s/$0/curl_cv_${0#ac_cv_}/g"') \
    $(git ls-files)

This only changed the prefix for 16 variables actually checked with
AC_CACHE_CHECK.