daemon: support backlog= listen parameter

daemon: support backlog= listen parameter

For -netd instances using multiple listeners, it's easiest for me
to configure all ListenStream directives in a single
systemd.socket(5) unit.  Unfortunately, this seems to force all
listeners to use the same Backlog= value, and juggling
multiple systemd.socket(5) units for a single systemd service
seems tedious.

So support setting the backlog= parameter on the public-inbox-*d
command-line on a per-listener basis; allowing us to override
the backlog value of inherited listeners and also to set the
backlog for listeners we bind ourselves.  This makes it possible
for non-systemd users to easily configure per-listener backlogs,
as well.