MINOR: server: prevent name collision with a default-server

MINOR: server: prevent name collision with a default-server

The previous patch has introduced the support for named default-server.
It is not possible though to declare a default-server with a name if it
collides with an already existing server instance.

This patch implements a similar check but on the other side : it ensures
that a newly created server instance does not collide with an already
existing default-server.

To implement this, a new function server_find_by_name2() is defined. It
is similar to server_find_by_name() except it also lookup in the named
default-server tree. Checks are performed on several places :
* _srv_parse_init() for a server parsing
* _srv_parse_tmpl_init() for a server-template parsing
* srv_update_server_name() for the CLI command "set server name"
* cli_parse_add_server() for the CLI command "add server"