MINOR: server: set default-server id to NULL

MINOR: server: set default-server id to NULL

Previously, default-server <id> was set to "default-server" static
string. With this patch, it is now set to NULL.

This has no noticeable impact, including on the configuration error
messages, as default-server ID is not used there. This is because
register_parsing_obj() is only used for standard server and
server-template instances, but not for default-server.

A nice side-effect of this change is that it's possible to free server
<id> member in srv_free_params(), which is less error prone (even more
with named default server future implementation).

This is a prealable for the future named default-server feature, as it
could conflict with a default-server explicitely named "default-server".
Also, for named default-server, it will be necessary to free <id>
member. Thus it's better to remove a static string reference to prevent
any issue with free() usage.