ctl: increase listen backlog to 5

When a control client reaches its timeout or is interrupted, the connection isn't
closed by the server immediately. So another connection attempts can be forbiden
with the error "OS lacked necessary resources". By increasing the listen backlog
such a situation is less probable to happen.

src/libknot/control/control.c

-/*  Copyright (C) 2019 CZ.NIC, z.s.p.o. <knot-dns@labs.nic.cz>
+/*  Copyright (C) 2020 CZ.NIC, z.s.p.o. <knot-dns@labs.nic.cz>
 
     This program is free software: you can redistribute it and/or modify
     it under the terms of the GNU General Public License as published by
@@ -36,6 +36,9 @@
 #define CTL_BUFF_SIZE		(256 * 1024)
 #endif
 
+/*! Listen backlog size. */
+#define LISTEN_BACKLOG		5
+
 /*! Default socket operations timeout in milliseconds. */
 #define DEFAULT_TIMEOUT		(5 * 1000)
 
@@ -208,7 +211,7 @@ int knot_ctl_bind(knot_ctl_t *ctx, const char *path)
 	}
 
 	// Start listening.
-	if (listen(ctx->listen_sock, 1) != 0) {
+	if (listen(ctx->listen_sock, LISTEN_BACKLOG) != 0) {
 		close_sock(&ctx->listen_sock);
 		return knot_map_errno();
 	}