MINOR: http: introduce the notion of http-hardening flags and sections

MINOR: http: introduce the notion of http-hardening flags and sections

The goal here will be to declare http-hardening sections which enumerate
controls that can be relaxed or enforced. The control names will be stored
in a table just like the http-violations, associated with a mask. They
will possibly permit to have names for sets that cover multiple bits. For
now only 32 bits are planned, covering both front/back and req/resp. This
should be sufficient for a start and we'll see once the limit is reached
what's the best way to proceed.

One difficulty here is to reason in terms of what is not permitted.
So the way to think about it is to associate each verb to an action
to be performed, such as reject-xxx, delete-xxx, adjust-xxx, block-xxx.