MINOR: http-conv: add hdng_rej_name to name the HTTP hardening reject rule

MINOR: http-conv: add hdng_rej_name to name the HTTP hardening reject rule

The hdng_rej_name converter takes a mask of HTTP_HRD_* flags such as returned
by fs.hdng.fixed or bs.hdng.fixed, keeps only the "reject" rules, and
returns the name of the lowest one, as found in http_hardening_kws[]. Since
a reject stops the processing, at most one is normally present. Other rules
are ignored, and the converter fails when no reject rule remains, so that
nothing is logged when nothing was rejected:

    log-format "... cli_hdng=%[fs.hdng.found,htonl,hex],%[fs.hdng.fixed,hdng_rej_name] ..."