Instead of having a single global check that all acceptors may race
towards, this check now happens on a per listen socket basis. For
sockets with a different inheritance behavior on a single system, we
avoid having the first connection dictate what may be inherited by
a connection socket from its listen socket for all the other listen
addresses.

At least on Linux, Unix-domain sockets DO NOT inherit options like
SO_{RCV,SND}TIMEO even though TCP sockets do. On the off chance that
even sockets of the same family could behave differently, like for
example a regular vs a loopback TCP session, this is done on a per
listen address basis. To avoid cache-acceptor coordination with the
acceptor worker threads of a given listen address, workers will
individually perform this check once and for all when the first
connection is accepted.

We also stay defensive in the event of a parameter change, just in
case a previous test would assume inheritance because the Varnish
parameter value would match the kernel default value. Once a mismatch
is observed for a given connection with a given socket, the inheritance
test is no longer performed needlessly for this combination.

A race still exists between acceptors from different thread pools for
a given listen address, but this race is identical to the previous one
based on the former global need_test variable.

Although the inheritance check leaks into struct listen_sock, it is
opaque so everything can remain contained inside cache_acceptor.c.

Some aspects of this change (including the clarification comments) are
from @mbgrydeland.

Refs #2722