MINOR: http: add the "set http-hardening" CLI command

MINOR: http: add the "set http-hardening" CLI command

"set http-hardening <name> <option> {on|off}" enables or disables an option
of a profile at runtime, including the "all" keyword to change everything
at once, e.g. in an emergency. Options may also be specified by the bit
number that appears in -dKcfg or in "show http-hardening". The option is
looked up like in the section parser so that a suggestion is made on typos.
The profile is only read by the other threads, so it is updated while the
threads are isolated, which avoids any locking on the fast path. The
command is restricted to the admin level.