launch: increase listener backlog to SOMAXCONN
launch: increase listener backlog to SOMAXCONN

Until today, we used a rather random backlog limit for AF_UNIX listener
sockets. However, it turns out the limit is used as literal limit on
linux to control the maximum number of pending connection attempts.
Since there is no fair queue sharing involved, all connecting users
share this single queue. Malicious peers can easily exhaust this limit
and cause connection failures for other peers.

Unfortunately, there is no known way to avoid this with AF_UNIX.
However, we can, at least, avoid the random magic number and stick to
the upstream limit. As it turns out, this is also what most other
AF_UNIX users do.

Additionally, document this situation. Fortunately, this will no longer
be an issue with bus1, since it employs fair queue sharing.

Signed-off-by: David Herrmann <fb53b2ddb8d141e0cb39d7c67c2f81b6bc2eb0f7@gmail.com>

bus1/dbus-broker