inpcb: Close some SO_REUSEPORT_LB races, part 2

Suppose a thread is adds a socket to an existing TCP lbgroup that is
actively accepting connections.  It has to do the following operations:
1. set SO_REUSEPORT_LB on the socket
2. bind() the socket to the shared address/port
3. call listen()

Step 2 makes the inpcb visible to incoming connection requests.
However, at this point the inpcb cannot accept new connections.  If
in_pcblookup() matches it, the remote end will see ECONNREFUSED even
when other listening sockets are present in the lbgroup.  This means
that dynamically adding inpcbs to an lbgroup (e.g., by starting up new
workers) can trigger spurious connection failures for no good reason.
(A similar problem exists when removing inpcbs from an lbgroup, but that
is harder to fix and is not addressed by this patch; see the review for
a bit more commentary.)

Fix this by augmenting each lbgroup with a linked list of inpcbs that
are pending a listen() call.  When adding an inpcb to an lbgroup, keep
the inpcb on this list if listen() hasn't been called, so it is not yet
visible to the lookup path.  Then, add a new in_pcblisten() routine which
makes the inpcb visible within the lbgroup now that it's safe to let it
handle new connections.

Add a regression test which verifies that we don't get spurious
connection errors while adding sockets to an LB group.

Reviewed by:	glebius
MFC after:	1 month
Sponsored by:	Klara, Inc.
Sponsored by:	Stormshield
Differential Revision:	https://reviews.freebsd.org/D48544

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