Fix the following issues related to the TCP SYN-cache:

• Let the accepted TCP/IPv4 socket inherit the configured TTL and TOS value.
• Let the accepted TCP/IPv6 socket inherit the configured Hop Limit.
• Use the configured Hop Limit and Traffic Class when sending IPv6 packets.

Reviewed by: rrs, lutz_donnerhacke.de
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D25909