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.

(cherry picked from commit efbf35caf9a3dcf9ead7bc07d140447e483a6bab)
Signed-off-by: Willy Tarreau <w@1wt.eu>
diff --git a/src/server.c b/src/server.c
index eb80104..5ccefcb 100644
--- a/src/server.c
+++ b/src/server.c
@@ -4404,6 +4404,13 @@
 		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)) {
 			cli_err(appctx, "MUX protocol is not usable for server.");