MEDIUM: http: add IPv6 support for "set-tos"

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.