libteam: add team_change_handler_register_head function

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
teamd: register debug callback at the start of callbacks list

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
teamd: allow to change debug level during run

Signed-off-by: Jiri Pirko <jiri@resnulli.us>