domains: merge domain_init() into domain_add()

domain_init() called at SI_SUB_PROTO_DOMAIN/SI_ORDER_SECOND is always
called right after domain_add(), that had been called at SI_ORDER_FIRST.
Note that protocols aren't initialized yet at this point, since they are
usually scheduled to initialize at SI_ORDER_THIRD.

After this merge it becomes clear that DOMF_SUPPORTED / DOMF_INITED
can be garbage collected as they are set & checked in the same function.

For initialization of the domain system itself it is now clear that
domaininit() can be garbage collected and static initializer is enough.