don't set listen(2) backlog on inherited sockets
By using the listen(2) backlog as-is when inheriting (from
systemd or similar), we can give the sysadmin more control on
controlling overload on a per-listener basis. For systemd
users, this means the `Backlog=' parameter in systemd.socket(5)
can be respected and configured to give certain sockets a
smaller backlog (perhaps combined with with per-listener
`multi-accept' parameter on sockets with the standard (huge)
backlog).
For sockets we create, just use a giant number and let the
kernel clamp it to whatever system-wide limit there is
(e.g. `net.core.somaxconn' sysctl on Linux).