MINOR: server: add the "http-hardening" server keyword

MINOR: server: add the "http-hardening" server keyword

The new "http-hardening" server keyword sets the HTTP hardening profile
that applies to the responses coming from the server and to the requests
sent to it. The server now holds a pointer to the profile, which may be
NULL if unset or explicitly set to "auto". It is inherited from
default-server, server templates and "from" like any other setting, and
may be set at runtime with "add server". The profile must have been
declared before.