BUG/MINOR: http: fix "set-tos" not working in certain configurations

s->req->prod->conn->addr.to.ss_family contains only useful data if
conn_get_to_addr() is called early. If thats not the case (nothing in the
configuration needs the destination address like logs, transparent, ...)
then "set-tos" doesn't work.

Fix this by checking s->req->prod->conn->addr.from.ss_family instead.
Also fix a minor doc issue about set-tos in http-response.
MEDIUM: http: add IPv6 support for "set-tos"

As per RFC3260 #4 and BCP37 #4.2 and #5.2, the IPv6 counterpart of TOS
is "traffic class".

Add support for IPv6 traffic class in "set-tos" by moving the "set-tos"
related code to the new inline function inet_set_tos(), handling IPv4
(IP_TOS), IPv6 (IPV6_TCLASS) and IPv4-mapped sockets (IP_TOS, like
::ffff:127.0.0.1).

Also define - if missing - the IN6_IS_ADDR_V4MAPPED() macro in
include/common/compat.h for compatibility.
DOC: remove the comment saying that SSL certs are not checked on the server side

Server certificate check was added in 1.5-dev13 but the comment on the "ssl"
keyword was not removed, leading to confusion about when to use it.
BUG/MINOR: deinit: free fdinfo while doing cleanup

Both fdinfo and fdtab are allocated memory in init() while haproxy is starting,
but only fdtab is freed in deinit(), fdinfo should also be freed.

Signed-off-by: Godbach <nylzhaowei@gmail.com>