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/core/socket.c

@@ -82,7 +82,7 @@ static void socket_init(Unit *u) {
         assert(u);
         assert(u->load_state == UNIT_STUB);
 
-        s->backlog = SOMAXCONN;
+        s->backlog = SOMAXCONN_DELUXE;
         s->timeout_usec = u->manager->default_timeout_start_usec;
         s->directory_mode = 0755;
         s->socket_mode = 0666;