BUG/MINOR: tcp: fix silent-drop workaround for IPv6

BUG/MINOR: tcp: fix silent-drop workaround for IPv6

As reported in github issue #1203 the TTL-based workaround that is used
when permissions are insufficient for the TCP_REPAIR trick does not work
for IPv6 because we're using only SOL_IP with IP_TTL. In IPv6 we have to
use SOL_IPV6 and IPV6_UNICAST_HOPS. Let's pick the right one based on the
source address's family.

This may be backported to all versions.

(cherry picked from commit ab79ee8b117dbb2c2872747e8119492e70506392)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 64300c5118f0e2cd40ccf1b6aa9d5f19ada0cdc9)
[wt: this is in proto_tcp.c in 2.2]
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 9650f63fb150e0f1c1bcb7012d59eff0ee61a868)
Signed-off-by: Willy Tarreau <w@1wt.eu>