Fix hung TCP sessions on shutdown

[jtl]

Summary

After r366857, TCP sessions are hanging on shutdown. This occurs when the shutdown script shuts down the network interfaces before init kills off remaining processes with open TCP connections. (For example, incoming SSH sessions are not killed off until after init is finished running the shutdown scripts.)

This change will kill off all TCP connections only when the script is shutting down all network interfaces. When a user is shutting down only some network interfaces, it is non-trivial to determine which TCP connections may be impacted.

[rrs]

Not exactly the best, but I think it fixes the immediate problem. From a long term perspective we may want to
look for a better solution :)

[tuexen]

It fixes the issue. For SCTP we would need something like sctpdrop. I can try to add that...

[tuexen]

When committing, please add a reference to https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=251540

[jtl]

In D27464#613656, @tuexen wrote:

When committing, please add a reference to https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=251540

ACK. Will do.

[jtl]

I just saw the discussion on the committers mailing list. First, it shows that @cy already has a proposed fix. Secondly, it shows that this is a larger issue (for example, netboot), which probably needs a different solution.

[cy]

See https://reviews.freebsd.org/D27459.