Limit of new TCP connections per source IP: configurable rate, drop only above the limit.

- default_settings.conf: new default_limit_new_tcp_connections_per_seconds_rate=30.
- main_ipv{4,6}.conf.sample: new limit_new_tcp_connections_per_seconds_rate
  (burst is twice the rate; invalid or empty values fall back to the default).
- ipt-firewall-server, ip6t-firewall-server: the rule no longer ACCEPTs SYNs
  below the limit (which bypassed the service rules, destination IPs and the
  HTTP limits); like the HTTP limits it now only logs and drops SYNs above
  the limit (--hashlimit-above). Fallback without xt_hashlimit: chain
  syn-multi-limit with a global limit.
- ip6t-firewall-server: fix --hashlimit-srcmask (128 instead of 32), drop
  only the given ports instead of all new TCP connections, hashlimit names
  syn_multi_v6*, add the can_use_hashlimit check.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

conf/default_settings.conf

@@ -6,6 +6,13 @@
 
 default_per_IP_connection_limit=250
 
+# ip6t-firewall-server,ipt-firewall-server:
+#     New (syn) TCP connections per second per source IP on the ports in
+#     'limit_new_tcp_connections_per_seconds_ports'. The burst is twice
+#     this value.
+#
+default_limit_new_tcp_connections_per_seconds_rate=30
+
 
 # -------------
 # --- Default Ports for Services out