socket: bump listen() backlog to INT_MAX everywhere

This is a rework of #24764 by Cristian Rodríguez
<crodriguez@owncloud.com>, which stalled.

Instead of assigning -1 we'll use a macro defined to INT_MAX however.

src/userdb/userdbd-manager.c

@@ -272,7 +272,7 @@ int manager_startup(Manager *m) {
                 if (r < 0)
                         return log_error_errno(r, "Failed to bind io.systemd.Multiplexer: %m");
 
-                if (listen(m->listen_fd, SOMAXCONN) < 0)
+                if (listen(m->listen_fd, SOMAXCONN_DELUXE) < 0)
                         return log_error_errno(errno, "Failed to listen on socket: %m");
         }