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.