BUG/MINOR: server: explicitly set "none" init-addr for dynamic servers

Define srv.init_addr_methods to SRV_IADDR_NONE on 'add server' CLI
handler. This explicitly states that no resolution will be made on the
server creation.

This is not a real bug as the default value (SRV_IADDR_END) has the same
effect in practice. However the intent is clearer and prevent to use the
default "libc,last" by mistake which cannot execute on runtime (blocking
call + file access via gethostbyname/getaddrinfo).

The doc is also updated to reflect this limitation.

This should be backported up to 2.4.

doc/management.txt

@@ -1466,6 +1466,10 @@ add server <backend>/<server> [args]*
   used to configure the server behavior. Also note that no settings will be
   reused from an hypothetical 'default-server' statement in the same backend.
 
+  Currently a dynamic server is statically initialized with the "none"
+  init-addr method. This means that no resolution will be undertaken if a FQDN
+  is specified as an address, even if the server creation will be validated.
+
   Here is the list of the currently supported keywords :
 
   - backup

src/server.c

@@ -4375,6 +4375,13 @@ static int cli_parse_add_server(char **args, char *payload, struct appctx *appct
 	if (errcode)
 		goto out;
 
+	/* A dynamic server does not currently support resolution.
+	 *
+	 * Initialize it explicitly to the "none" method to ensure no
+	 * resolution will ever be executed.
+	 */
+	srv->init_addr_methods = SRV_IADDR_NONE;
+
 	if (srv->mux_proto) {
 		if (!conn_get_best_mux_entry(srv->mux_proto->token, PROTO_SIDE_BE, be->mode)) {
 			ha_alert("MUX protocol is not usable for server.\n");