MINOR: cfgparse: dump the "http-hardening" keywords with -dKcfg

MINOR: cfgparse: dump the "http-hardening" keywords with -dKcfg

The keywords of the "http-hardening" section are not registered via the
cfg_kw_list mechanism but through their own table in http.c, so they were
missing from the output of "-dKcfg". Let's have cfg_dump_registered_keywords()
report them by calling dump_http_hardening_kws() once the other sections
were dumped. The function also reports the option numbers for pure options
(those which are not composite).