MINOR: server: prepare parsing for dynamic servers

Prepare the server parsing API to support dynamic servers.
- define a new parsing flag to be used for dynamic servers
- each keyword contains a new field dynamic_ok to indicate if it can be
  used for a dynamic server. For now, no keyword are supported.
- do not copy settings from the default server for a new dynamic server.
- a dynamic server is created in a maintenance mode and requires an
  explicit 'enable server' command.
- a new server flag named SRV_F_DYNAMIC is created. This flag is set for
  all servers created at runtime. It might be useful later, for example
  to know if a server can be purged.
diff --git a/src/cfgparse-tcp.c b/src/cfgparse-tcp.c
index e7868e6..a15a110 100644
--- a/src/cfgparse-tcp.c
+++ b/src/cfgparse-tcp.c
@@ -282,7 +282,7 @@
 
 static struct srv_kw_list srv_kws = { "TCP", { }, {
 #ifdef TCP_USER_TIMEOUT
-	{ "tcp-ut",        srv_parse_tcp_ut,        1,  1 }, /* set TCP user timeout on server */
+	{ "tcp-ut",        srv_parse_tcp_ut,        1,  1,  0 }, /* set TCP user timeout on server */
 #endif
 	{ NULL, NULL, 0 },
 }};